I believe that due to a myriad of reasons (going open world, appealing to a wider audience & possible lack of time/manpower) and unlike its predecessors The Witcher 3's itemization aspect ended up borrowing heavily from another, popular genre of open world pseudo-rpgs: massive multiplayer online rpgs.
The requirements and goals of itemization systems used in massive multiplayer games that are designed to keep the customer playing (& paying) for thousands upon thousands of hours and where the core of the experience consists of the mindless grinding of enemies in an endless hamster wheel-spinning pursuit of procedurally-generated loot/power/wealth in order to outrank other players are perhaps not the best fit for a game like The Witcher 3. TW3 is a much shorter & heavily story-focused single player game where the story/setting/lore heavily imply a certain amount of loot poverty, loot realism, a certain uniqueness to items and a pre-established primary means of income for the protagonist (witcher's work/quests), and it's a game where nobody stands to profit from wasting the player's time.

What this mod does is remove that upper layer of mmorpg-style design considerations that introduced random and incoherent procedurally generated loot and looting as a timesink, all the while preserving the existing lower layer of core designs that provide a handcrafted itemization experience. It does so by following a rules-based approach to changing the game's itemization systems, with every systemic change having a clear gameplay experience goal in mind while preserving as much of the original game that was set in stone as possible, avoiding making a multitude of small & arbitrary value changes.

This translates to a truly different (& significantly lore-friendlier) itemization experience for the player while still being able to rely on things like guides&wikis to see where to find specific items, what stats gear has or how much a quest pays, and not needing to "re-learn" the game. 

Gear:
- Incoherent gear power differences: in the vanilla game you can obtain a unique legendary ancestral relic sword as a reward from a particularly tough quest and it does only a fraction of the damage and is less valuable than the common small blackjack that drops off a shirtless bandit a few of hours later. There is a clear incongruence between the way the game presents/makes the player expect a given gear item's power/value to be, and what it is in practice. Fixed by Items Level Normalized (by scaling melee weapons&armor to the player's level) so that relative power differences between gear items are preserved no matter the player's level or stage in the game (higher quality gear is better). Proper gear progression is still preserved as the higher quality items that you find at a later stage in the game were already defined as having better bonus stats than the equivalent earlier ones. Its included No Higher Level Items in Shops option makes sure shops don't put items for sale while their original level would still be above the player's current level, so you can't buy endgame items at the beginning. If you manage to kill a very high level contract monster and get a powerful gear reward you should be able to use it unlike in the vanilla game where you might not meet its level requirement. (FCR3 further scales bolt damage to the player's level)

- Generic indistinguishable gear statistics: common weapons get a random assortment of bonus stats randomly picked from a generic list of possible bonus stats. When you've already collected a bunch of novigrad, dorian etc longswords with igni sign boost then finding the rare relic that always has that bonus stat doesn't mean anything as you've already seen that stats on dozens of swords before. This is fixed by No Random Gear Stats & Stat Values so gear items don't get randomly added bonus stats, they only have the ones they were predefined with. This means common gear should only have few or no bonus stats (and of a common variety), and the rarer and more powerful bonus stats are only seen on the higher quality relic/witcher items. This makes higher tier gear feel significantly different from common gear and make those gear items with predefined bonus stats feel more characteristic/unique. The feeling of uniqueness is further improved by No Duplicate Relics that makes it so you don't find duplicate copies of relic gear as loot any more(or horse gear, another type of item you only need one copy of)..
 
- Bad item stat lottery: in the vanilla game kill a specific contract boss monster and it drops a relic with a +1% crit chance bonus, reload the game and kill that boss monster again and it drops the same relic only this time it might have a +15% crit chance bonus. This kind of loot lottery is done in mmorpgs where a given enemy can drop a variety of different items/parts of a set to encourage the player to play through the same content again(to keep him busy) and perhaps encourage him to engage in trade with other players when he gets duplicates/something he doesn't need. TW3 is a single player game, there is no point to that kind of lottery. No Random Gear Bonus Stats & Stat Values together with Item Levels Normalized fix it by making it so that you always get the best possible version of any item you pick up (stats are maxed out instead of randomly picked between min and max vales), so that obtaining that item always feels as rewarding as could be.

- Lack of variety in gear progression: the few witcher gear sets are far better than any other gear and available almost from the beginning of the game, that means that your gear progression will ignore 95% of the gear items in the game and be entirely front-loaded, relegating most non-witcher gear to things that you merely collect or sell. This mod tries to further spread out your gear progression so that all the tiers of gear and a larger variety of gear are a part of your gear progression. Crafting Witcher & Relic Gear fixes this by pushing the crafting of relic & witcher gear to later in the game (master craftsmen required) so that you will have to rely on common/master/magic tier items as well as occasional relic loot until then. Together with Lore Friendlier Loot which makes it so weapons&armor are no longer randomly spawned as loot this means that when you are given a sword that is said to be fit to slay a god with, it should feel appropriately powerful, unique and valuable compared to what you had at that stage in the game. (FCR3 also further nerfs witcher gear stats).


Loot:
-Incoherently placed loot: in the vanilla game you can find diamonds inside the sacks of a starving peasant's hut. Reload the game and there might be a rare witcher recipe instead, or a relic sword, or a broken rake. Most of the instances of rare items the player finds as loot are randomly generated from generic loot lists(made by an automated tool most likely). Reduced Lore Friendlier Loot makes it so you only find those kinds of rare & valuable items as loot in the places where they were always meant to be in the vanilla game (likely placed there by hand by a developer). This means you shouldn't find loot that feels out of place any more. Smelly peasant huts shouldn't hold more than junk or misc category items. This also contributes to making gear feel unique as enemies don't drop junk tier weapons and low level gear also doesn't spawn as random loot any more, so when you do happen to obtain a new weapon/armor as loot it is more noticeable and feels more special. The removal of food category items from randomized loot lists and its reduction in the handplaced loot lists also means that in places like Velen you will feel a clear famine compared to a place like Toussaint.

-Not enough visual clues for the looting process: while some players do loot every single barrel, crate and washtub that their witcher senses button tells them is interactive, not all do. The latter risk missing on important loot by ignoring such trash-looking containers. Rare Items Glow adds the quest item blue glow effect that is visible without having to activate your witcher senses to any containers that holds a rare item, defined as the type of item that you can't use more than one copy of and might as well be unique (recipes, schematics, books, relic/witcher gear, gwent cards, horse gear, quest item) so that the non-ocd player doesn't miss out on anything important. Loot Bags Glow similarly adds a yellow sparkle effect to enemy loot bags so that you don't have to rely on witcher senses or the minimap to figure out where the disappeared corpse's loot is.

-Timesink unintuitive trashcan looting: while looting trashcans and baskets might be as much of a time honored rpg-padding tradition as throwing endless waves of rat-level enemies at the player, tw3 has nearly 200 hours of quality content. There's no need for padding.  And unlike a bethesda game where you can assume that every single basket or trashcan is lootable -likely containing only trash- and so where all trashcans can be treated equally (meaning equally ignored), in TW3 only a random assortment of those decorative-looking containers are lootable (perhaps made so by an automated tool) and thus they are made to seem as important as any other lootable container when you are forced to resort to using your witcher senses to figure out which containers can even be looted to begin with. No Decorative Container Looting saves the player from hours upon hours of witcher-sense vision trashcan looting by making it so that only chest-like containers are lootable by default, the kind of containers that the player should be able to visually recognize as lootable  by sight without having to resort to witcher senses (as nearly all chests in the game were lootable to begin with, unlike trash/decorative containers). Decorative-looking containers that happen to hold a rare/unique item are still lootable, however the player doesn't need to worry about using witcher senses to spot those as they would be glowing blue already due to containing a rare item (the work together like lego pieces). Reduced Lore Friendlier Loot also reduces the amount of time wasting in meaningless looting by making it so pack-type monsters (that you will kill thousands of) drop loot less often (unlike their alpha varieties, nests and solo monsters). No Junk Weapon Drops also makes you spend less time looting as human enemies no longer drop their weapons if they are below relic quality, and thus also significantly contribute to making it so that the few weapons you do find feel special. No Junky Junk Drops further reduces the amount of time spent looting as it makes sure enemies no longer drop junk category items that aren't jewelry (as junk items are now the most common type of randomized loot). Shops Don't Buy Junky Junk further spares the player's time by discouraging him from the blind robbing of peasant huts by making it so the type of loot that remains inside their lootable chests (trash junk) is worthless for selling (but is still useful for disassembling/acquiring crafting ingredients at a discount). Greyed out junky junk further makes it easier to ignore containers that only carry worthless/junky junk items. Finally Well Stocked Craftsmen makes it so the player will never again have to scour the countryside trying to find the missing piece of twine or herb he needs to craft his recipe which no merchants seem to carry for no reason, as they now will always stock the common apropriate common ingredients required for the recipes/schematics the player knows.

 
Economy:
-Incoherent prices: the king's generous reward for rescuing his princess might be 1000 crowns, but with vanilla shop prices that can merely afford Geralt a few days of boozing, yet the rusty sword dropped by the shirtless bandit can also be sold for more than that, which is about the same as you can sell a unique relic sword for. Buy a weapon repair kit from the blacksmith for 50 crowns or pay him to repair the sword himself for 10 times that amount.  Cheaper Lore Friendlier Shop Prices makes prices align with what's said in dialogs and practiced in quests by drastically reducing shop selling prices (removing their markups), this means that the price you see for an item in your inventory is the same price you can buy it for. Deleveled Gear Prices makes it so gear levels no longer exponentially increase their buying, selling & servicing prices, this means the common bandit's weapon won't be worth a fortune just because you picked it up at a high level and it also preserves the original price differences between items(it should be much cheaper than a relic sword). Cheaper Repair makes it so paying for repairs is valid alternative to repair kits when you're lucky enough to be near a smith/armorer. 

-Bloated income: the vanilla game's economy deliberately bloats the average player's income as he progresses so that by the halfway point in the game he can already afford to buy anything he sees. That's why it even introduced moneysinks in the expansions (runewright, corvo bianco). If you can buy everything then money is no longer a concern or interesting mechanic. In order to make money meaningful again, Reduced Lore Friendlier Loot drastically reduces how much money the player makes off loot, while also making it so there are more purposeful things to buy now (what the player no longer easily obtains as randomized loot -recipes- he might now want to buy). Its reduced creature loot means you will no longer be stacking monster parts by the hundreds while all the game's recipes and schematics call for only half a dozen of them (yet you should still acquire them almost as early as before) No Junk Weapon Drops & Deleveled Gear Prices cause the biggest reduction in what used to be the biggest income for the player( selling dropped weapons) by making it so enemies don't drop common weapons any more and also making it so their prices no longer drastically increase with its level.

-The Merchanter/peasant robber: the story/lore/dialogs clearly clearly establish the protagonist's primary means of income as his profession: a witcher. Yet in the vanilla game monster contract money rewards are inconsequential compared to what you make off selling the loot you get off peasant huts and bandits. By significantly reducing the other means of income this mod makes it so quest rewards are once again the primary moneymakers, where haggling over a few dozen coins more for a contract should actually feel like it makes a difference.
This should encourage the player to seek out new contracts to wrangle for his income rather than looting peasant huts, attacking random packs of wild animals or bandits.

-No cost of living, infinite health potions: in the vanilla game alcohest and dwarven spirit drop plentifully as loot, and are used to automatically refill your alchemy items making those loot drops akin to health potion drops and the cost of alchemy refilling or obtaining health potions never a concern. Simple Alchemy Refilling increases the cost of refilling your alchemy items and then Reduce Lore Friendlier Loot makes it so alchemy ingredients such as alcohest no longer constitute randomized loot, so you will often be buying it. The added cost of alchemy can make the player take into account how much the contract is going to pay before he decides how much to spend on the apropriate alchemy items. This makes money more meaningful as you do need it to craft the alchemy items needed to survive tougher fights and alchemy refilling a more involved process. No Infinite Bolts further increases the player's cost of living/operation by making it so you no longer have infinite bolts, having to rely instead on those you loot or perhaps even purchase or craft, thus getting more involved with their mechanics and differences.

Removes gear level requirements and hides UI elements that display their level.
Removes weapons&armor's bonus stats that were randomly added from a generic stat list, as opposed to the bonus stats that are a part of that item's definition.
Maximizes the actual values of weapon/armor's predefined stats instead of randomly picking between their predefined minimum and maximum values.
NewGame shops don't sell NewGamePlus dlc set items. NewGamePlus shops don't sell NewGame dlc set items.
Scales weapons&armor to match the player's level using the game's own autogen scaling system and correctly calculates the values for bludgeoning weapons as well. For crossbow level scaling, the vanilla crossbow level range system is extended up from the vanilla ">190% attack power->lv31" max all the way up to "attack power >=300% (the max crossbow attack power in the vanilla game)->level50" (50 is the NG endgame level where geralt stops gaining levelups stats). Bolts are all fixed level in vanilla and already damage-scale with player level in FCR3.
Hides a shop's weapons, armor & crafting schematics while they are more than one level above the player's level. Exceptions made for Countess Mignole's temporary shop.
Increases the required craftsman level for Master for all Witcher&Relic Gear that required less than that (configurable). To compensate for the delay in first acquiring the warrior's leather jacket and viper swords their stats are boosted to the NG+ version(and the NG+ version accordingly). Schematics also provided for witcher-quality versions of hunter boots/gloves/pants.
Prevents item levels from factoring into their buying, selling & servicing(repair, disassemble, crafting, socket adding, upgrade removing) prices. Whenever the game calculates the price for buying/selling/servicing a weapon or armor it uses a deleveled/lv0 version of that item.
Once you've obtained a relic-quality sword/armor (or horse gear) you won't find any more copies of the same item as loot or in shops.
Alchemy items now can be individually refilled using straight from the alchemy menu using either their corresponding base, base+herbs or full recipe(configurable). You can choose which tier of an already unlocked recipe to refill /how much to spend on a refill (you can only refill lower level versions of your items if their refilled amount would be greater than the current amount in your inventory). Meditation automatic refilling can be disabled, set to refill the same as vanilla(one alcohol item refills all your alchemy items), or set to Simple Alchemy Refill where level and type-appropriate corresponding base ingredients are used to automatically refill your alchemy items (one level-appropriate alcohol used to refill your potions/decoctions, one level-appropriate bomb base used to refill your bombs etc).  You can configure the maximum number of charges each alchemy item type can hold. You can configure whether or not you need to be near a campfire/witcher bed/alchemy table to be able to refill your alchemy items. Alchemy table now only boosts the number of charges of alchemy items that were already fully charged. Bottomless caraffe replaces alcoholic base ingredients in potion&decoction recipes.
Oil items now need to be refilled same as other alchemy items.
GUI elements added to the alchemy menu to let the player know if he has any charges left of a currently selected recipe, or of a different level version of the same item.
Alchemy recipe base ingredients can now be downgraded to their inferior versions, and 3 of them used to craft a superior version.
The price of Alcohest is switched around with that of Dwarven Spirit as Alcohest is an enhanced-level alchemical base ingredient and thus should be more expensive than its basic-level version: Dwarven Spirit. This also preserves the cheapest price for refilling your alchemy items that is mentioned in game dialogs. 
Containers with quest items, new recipes/schematics/books/gwent cards, relic or better equipment, horse gear get the blue sparkle effect without witcher senses.
Loot bags get the yellow sparkle without witcher senses.
Randomized container loot lists only retain their junk(junk, jewelry, furs) and misc category(coins) items. Handplaced loot lists are untouched.
Duplicates of already obtained relics & horse gear found as loot are replaced with a proportionate amount of coin. Configurable.
Dye trader now sells dye recipes as their locations were strictly random.
Yoana now sells the few gear items that existed only in randomized loot lists (relic armors) Light armor 01r Light armor 02r ,Light armor 03r Light armor 04r Light armor 06r Light armor 07r Light armor 08r Light armor 09r Medium armor 01r Medium armor 02r Medium armor 03r Medium armor 04r Medium armor 05r Medium armor 07r Medium armor 10r Medium armor 11r Heavy armor 01r Heavy armor 02r Heavy armor 03r Heavy armor 04r Heavy armor 05r Boots 012 Boots 022 Boots 032. To preserve gear progression her shop services are delayed to a later stage in the game (whens he starts offering crafting services).
Novigrad book merchant now sells the books that existed only as random loot: Beasts vol 2 Cursed Monsters vol 2 Draconides vol 2 Insectoids vol 2 Necrophage vol 2 Relict Monsters ol 2 Specters vol 2 Ogres vol 2 lore_tyromancy lore_nilfgaardian_transport_orders lore_unfinished_war_annals
White Orchard Griffin merchant sells a saddle that existed only as random loot Horse Saddle 1v3
Random container loot lists now only draw 1 random item per loot list.
Monsters no longer drop raw meat. Non hostile animals (deer, rabbit, pigs etc) & boars still do.
Monsters & monster nests no longer drop non-monstrous crafting ingredients (panthers still drop fur scraps), non-monstrous alchemy ingredients & upgrades(runes/glyphs).
Monsters don't drop junk any more.
Contract Monsters no longer drop relics. Their monster hunt quest givers instead add the appropriate schematics to their quest rewards/payment.
Monsters & animals don't drop more than 1 copy of each item(if a bear previously dropped 1 bear fat and 3 claws he now drops 1 bear fat and 1 claw). This means you should no longer be able to easily accumulate large amounts of valuable monster loot that you don't really need.
Common pack-type monsters(nekkers, wolves, dogs, ghouls, sirens etc) and non-hostile animals only drop at most 1 item per monster. You should still be able to acquire the rarer monster ingredients required for alchemy/crafting as early as before.
Packs of monster now typically only have 1 monster drop loot per pack (configurable).Also means that if you want to hunt for pack monster ingredients you're better off looking for their alpha versions and nests instead.
The loot entries that are removed from monster loot lists are in fact replaced with an entry for a NoShow/NoDrop item set with the same priority as the entry it's replacing in order to preserve the vanilla relative drop rates for the remaining items (ex: drowners are still 10 times more likely to drop a drowner brain than a monstrous hair).
Food is preserved in food-shaped containers, but their amount reduced to 1 per loot list/container.
Shops pay 0 crowns for junk category items that aren't tagged as precious(jewelry), valuable(drugs) or collectible(fossil). Configurable.
Enemies no longer drop junk category items that aren't tagged as precious(jewelry) or as a quest item.
Icons for junk items that aren't tagged as precious(jewelry), valuable(drugs), or collectible(fossils) appear greyed out. Same as already read books or recipes.
Enemy loot bags get the yellow sparkle without witcher senses. The effect gets paused during cutscenes.
Adds the quest item glow/sparkle effect to any container that holds a recipe/schematic/book you haven't read yet, a gwent card that you can add to your collection, horse gear, a weapon or armor of relic/witcher quality, a quest item. The effect temporarily pauses during cutscenes. Configurable.
Enemies don't drop their equipped weapons as loot unless they are artifact/witcher-quality or quest items.
Disables containers based on what they look like (as in what 3d model/mesh a container uses), so pretty much all containers that use models that don't look like they should hold anything of value to a witcher such as sacks, crates, bundles, barrels, baskets, cupboards, commodes, shelves, cabinets, washtubs, pots, cloth piles, tables, dressers etc are no longer interactive. Exceptions are made for when a decorative-looking container is actually important such as being connected to a quest/poi/dialog line on opened, glowing red, locked, so that you don't end up with broken quests or POIs you can't clear. Exceptions are also made for when a decorative-looking container holds a rare item - defined as a quest item, a relic or better piece of gear, a recipe/diagram/readable item you haven't read before, a gwent card you can add to your collection, horse gear- as in the kind of item that you only really need one copy of.
Craftsmen will always carry the appropriate nonmutagen/nonmonstrous/nonbuckthorn/nonwhitegull/noncorvobianco common ingredients required for your recipes.
Merchants sell items at their base price, without the bloated sale markup.
Reduces the cost of repairing items at a blacksmith/armorer down to 1 crown per durability point. Configurable.
Disables the use of infinite crossbow bolts unless a quest requires them.
Reduces Corvo Bianco upgrade costs to 2600
Reduces runewright upgrade costs to 5400.
Vanilla bugged "Missing in Action" quest in White Orchard that previously paid nothing now pays 20 crowns.
Adds "Blunt Bolt schematic" to most craftsmen to compensate for the absence of the infinite use bolt and scarcity of bolts & bolt schematics in WO+Velen shops.
NewGamePlus, HoS-only and B&W-only modes start you off with the same gold amount that NewGame does (250).
NewGamePlus, HoS-only and B&W-only modes start you off without any alchemy ingredients, crafting ingredients, runes/glyphs, unequiped geralt & horse gear (torches, potions/bombs/oils, unique misc items, equiped weapons/armor/bolts and the starting gear set excluded).

Simple Alchemy Refill
Simple Alchemy Refill: On/Off (Default: on)
^Whether or not the mod is active

Meditation Automatic Refill: Off/Simple Alchemy Refill/Vanilla (Default: Simple Alchemy Refill)
^Whether automatic refilling of alchemy items during meditation is disable, set to use the apropriate ingredients for the types of items and their levels, or to be as cheap as vanilla

Meditation Automatic Refill Repeats: On/off (Default: on)
^Whether or not an automatic refill repeats until your items are full (in case you're using increase max quantity multipliers)

Meditation Refill Requires Campfire (Default: On)
^Whether you need to be near a campfire for meditation automatic refill to work

Alchemy Menu Cooking Requires Campfire (Default: Off)
^Whether you need to be near a campfire for alchemy menu brewing to work

Bomb/Potion/Mutagen Potion/Oil Refill Mode: Base / Base+ Herbs /Full Recipe (Default: Corresponding Base)
^Whether refilling those items requires their formula's corresponding alchemical base, base+herbs, full recipe (no mutagens).
(Default: Refill with corresponding Base as that already feels expensive enough when used the loot-reducing mods.)

Bomb/Potion/Mutagen Potion/Oil Quantity max multiplier: 1-10 (Default: 1)
^1x = no change, 2x you can stack twice as many charges of an alchemy item as before (how much you get per refill is unchanged) etc.

Oil Refill: On/Off (Default: On)
^whether or not oils require refilling. Off= infinite oil same as vanilla.

Oil lv1/2/3 refill quantity: 1-10 (Default: basic = 1, enhanced = 2, superior= 3)
^How much oil you get when you refill an oil. Default oil refill quantity= oil level.

Add missing recipes: on/off (Default = on)
^Whether or not the game automatically adds missing recipes for alchemy items you already have (added during loading).
(Default = on as both New Game and the HoS/B&W-only starts have omissions and start you off with alchemy items you don't have the recipe for (ex:Tawny owl) but that you're expected to be able to refill from the start anyway and you can only refill an item from the alchemy menu if you first have its recipe)


Crafting Witcher & Relic Gear Requires Master Craftsmen
The mod menu lets you configure whether the mod is enabled or disabled and set the item quality level (common/master/magical/relic/witcher) starting from which Master Craftsmen are required for crafting it.
If you disable this mod then I strongly suggest you delete its submod that boosted the early craftable witcher gear (\modlegoupgradedstartingvipergearlego and \dlclegoupgradedstartingvipergear lego)

Cheaper Repair
Enable/disable the mod and configure how much it costs to repair 1% durability damage (from 0 -free- to 10 crowns per durability damage %) and set it separately by item quality type (common/master/magical/relic/witcher gear).

Shops Don't Pay for Junky Junk
Enabled/Disable the mod and set the multiplier for junk sale prices (0= no pay, 1= full vanilla price). Ex: set it to 0.01 to sell junk items for 1 gold coin a piece.

No Duplicate Relics
No Duplicates (Enabled/Disabled): Controls whether the mod is active or not. If you had this off and then toggled it back on then you might want to reload your save (so the mod can scan your inventories for already acquired relics/horse gear during loading).

Replacement Gold Value % (0.01-100%): How many coins a duplicate item gets replaced by.
54 (default) = the item gets replaced by 54% of its original (unleveled) value in gold.

Rare Items Glow
Lets you enable/disable the mod and set whether only rare items make containers glow, or whether any lootable items make containers glow( No Decorative Container Looting still applies). It's best to save&reload after you change this mod menu settings.

Reduced Pack Monster Drops
Mod menu lets you turn the mod on/off and set drop rate (1/X) for every pack monster type:
1 = every monster you kill will drop loot (1/1), 2 = every second monster you kill will drop loot (1 in 2) etc, 0= no drops.

First you gotta locate your The Witcher 3 game install location. Usually it's somewhere like "C:\GOG Games\The Witcher 3 Wild Hunt GOTY" for Good Old Games versions of the game or "C:\Program Files (x86)\Steam\steamapps\common\The Witcher 3" for Steam versions of the game.

Mandatory:
Script Merger
Download, run/install it, run what it installed to be sure it's working.
Script Merger Unofficial Patch:
Download, run/install it, run what it installed to be sure it's working.

Optional but highly recommended:
TW3 Mod Manager
Download, run/install it, run what it installed to be sure it's working. Installing it all to some place like C:\TW3Modding\ should work.
Mod Limit Fix
Download, run/install it, run what it installed to be sure it's working
Enable the Debug Console: https://www.nexusmods.com/witcher3/mods/3375. Press the keys on your keyboard until you figure out which one activates the console.



After that you can install most mods by downloading them manually (VORTEX NOT RECOMMENDED WITH TW3 MODS, IT WILL EVENTUALLY CAUSE PROBLEMS). Extract their contents somewhere (rightclick on the downloaded mod file->extract) and then place their files&folders in the right places in your game install yourself OR let TW3 Mod Manager extract and place them correctly for you (Recommended).

If you're not using TW3 Mod Manager you need to make sure all of your mod's \modxxx folders end up directly inside your <TW3 game install>\Mods folder (or <TW3 game install>\mods, wherever your other mods are). If there's no mods folder in your TW3 game install folder (TW3MM would've created one for you) you can create one by rightclicking on some empty space there -> "New" -> "Folder" and write Mods as its name. That means your \modxxx folders ends up as <TW3 game install>\Mods\modxxx. TW3MM would place it there automatically for you so you wouldn't have to.

If your mod has \dlcxxx folders they similarly need to be directly inside your <TW3 game install>\dlc folder (or <TW3 game install>\DLC wherever your other dlcs are). They should end up as <TW3 game install>\dlc\dlcxxx ( never <TW3 game install>\dlc\dlc\dlcxxx !). TW3MM would do this automatically for you.

Your mod's mod menu file xxx.xml would have to end up as <TW3 game install>\bin\config\r4game\user_config_matrix\pc\xxx.xml. Again, TW3MM would do this automatically for you. If you happen to have more mod menus tham what can fit on the mod menu list you can easily edit & group mod menus together into subgroups yourself using something like notepad++, example: 1-> 2 (that mod menu was placed in a "combat" subgroup, example from Menu Strings).

If your mod has custom keybinds you probably have to add them to your <My Documents>\The Witcher 3\input.settings file using some editor like notepad++. Once more, TW3MM will usually do this automatically for you.

After your mod's files & folders are in the right places you need to run patched Script Merger and merge any conflicting files that can be merged together (that means both .ws scripts and .xml bundled files).

Sadly, not all mods will automatically merge together, most probably won't (though all LEGO merge automatically with each other, obviously), but some unresolved conflicts can be solved with a little manual help:



The bottom half is the output/result of your merge, the yellow <Merge Conflict> line is the conflicting line that SM can't figure out automatically. 
Right click on the merge conflict and you get some simple options:


^There you picked the line from mod 2, but you can also pick the lines from both mods, in any order:


Or you can even write your own code in there. Of course knowing which lines to choose or how to make them work together does require you to somewhat understand what the code does. When in doubt you might try asking the concerned mod's authors & users for help in the comments sections.

However keep in mind that there's no guarantee that any 2 mods can be effectively merged together, nor is there any guarantee that 2 mods are working well together even if they merged automatically and the game seems to be running ok.

That's why to be extra safe you should carefully read each mod's install instructions to make sure there aren't any special instructions or requirements for the mod to work (some mods might require other mods to work such as the unification/community patch or shared imports) or incompatibilities, and then merge & run the game after each new mod install to make sure it's running well.

\Mod\Bundle\dlc\bob\data\gameplay\container_definitions\ep2_loot_food__bowl_a.w2ent
\Mod\Bundle\dlc\bob\data\gameplay\container_definitions\ep2_loot_food__bowl_a_shadow.w2ent
\Mod\Bundle\dlc\bob\data\gameplay\container_definitions\ep2_loot_food__foodplate_a.w2ent
\Mod\Bundle\dlc\bob\data\gameplay\container_definitions\ep2_loot_food__foodplate_f_.w2ent
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