• For JSON implementation: create NEW folder of your choosing somewhere in meshes folder. Let's say "MyFancyAssaultCarbMags" inside "meshes\weapons".
• Open the weapon model you want a magazine for, in Nifskope. Let's say Assault Carbine.
• Select NiTriStrip\mesh you want to be a magazine in the list on the left and press Ctrl+C (or Copy Branch via dropdown menu).
• Open template.nif file (located in the archive). Navigate to "Parent" node. Click on it. Press Ctrl+V (or Paste Branch via dropdown menu).
• Copy all meshes you want to be a "magazine" like that (like, maybe you want rounds in it, or maybe mag is made of dozens of meshes, you know what I mean)
• Now right-click on Parent node and select Transform->Edit. And move it around until mag is rougly in the center of the scene.
• Navigate to bhkCollisionObject-bhkRigidBodyT->bhkBoxShape. Select it. In block details you will find Dimensions parameter. Edit it by double-clicking on values.
• Adjust those values until collision shape roughly fits the magazine size. Adjust Parent node transforms if needed.
• Save this new model into that folder you've created (meshes\weapons\MyFancyAssaultCarbMags), with one of the following names:
  

0.nif = Empty mag
1.nif = Extended Empty mag For guns that have extended magazine. If 1.nif absent, 0.nif mag will be used automatically
0_F.nif = Partial\Full mag (well, just a mag with few rounds visible to indicate it's not empty).
1_F.nif = Partial\Full extended mag.

• NEW! Some weapons have alternative skins as mods\attachments, for those cases you might want to add these files together with the ones above (more info in JSON part down below):
  

0b.nif = Empty mag with alternative skin
1b.nif = Extended Empty mag with alternative skin
0b_F.nif = Partial\Full mag with alternative skin.
1b_F.nif = Partial\Full extended mag with alternative skin;

Having partial mags is not 100% necessary, since the partial mags are entirely optional. If they are missing but user wants to have partials, mod will automatically use EMPTY (0.nif for normal, 1.nif for extended, if it exists) mag model for the partial mag drop. You can always add partial models later, even AFTER the mag was dropped with empty one and game was saved. Mod will seek for it, and automatically replace it with correct mag after the fact.


That's it, that's the hard part. Now JSON!

Create new JSON file (it's just a .txt with .json extension instead) inside "data\config\B42Dropmag" folder. Name doesn't matter, but I'd recommend using some specific name that would be hard to overwrite by mistake for somebody else. And fill that file using this template (or one of the examples included in the archive):

[
{
"mod": "ModName.esp OR ModName.esm",
"form": "FormID goes here (FORM, not EDITOR)",
"folder": "PATH\\TO\\MAGS\\GOES\\HERE"
}
]


NEW! Now Dropmag supports attachments that make magazines look different (that are not clip size chaning ones), like skins. For example, our Assault Carbine might have an attachment that adds golden skin to it.
And that skin changes magazine to golden as well. So new OPTIONAL (you can totally ignore it) parameter was added into JSON to allow those changes effect dropped magazine: "alt": "mod number"
It takes the attachment number as input. You can check it either in geck, or in the game in pipboy attachments menu. If it's first, it's 1, if second, then 2, you get the idea. So if our "skin" attachment is in 3rd slot = we can add it into the "alt" parameter, and now every time we apply that attachment/skin to our carbine, it will drop fancy golden mags.
Now let's fill the template with all of our stuff from above. Mod name? FalloutNV.esm. Weapon FormID = 0008F21E. We cut all zeroes in the beginning and get 8F21E. Path? meshes\weapons\MyFancyAssaultCarbMags. But use \\ instead of \, so JSON doesn't poop itself. Golden Skin fancy mod slot number = 3 (OPTIONAL) And this is what we have:

[
{
"mod": "FalloutNV.esm",
"form": "8F21E",
"alt": "3",
"folder": "meshes\\weapons\\MyFancyAssaultCarbMags"
}
]


And there you go, your mags are ready.
Alternatively, if you don't want to mess with JSON, or don't even need it, you can use EditorID implementation instead:
Mag creation process is exactly the same as for JSON, but you have to place those mags inside:
"meshes\B42\Dropmag\IDs\Your Weapon EditorID" folder. And that's it. No extra steps.
Using example above, Assault Carbine, it'll be "meshes\B42\Dropmag\IDs\WeapNVAssaultCarbine". That's how vanilla weapons are implemented in the mod itself as well.


Now, if you are not familiar with EditorIDs, FormIDs, how ammo works in New Vegas, etc: keep reading

So, what the hell is EditorID? It's basically a special name of any object, used by the GECK. It's always in the first column when you check stuff in GECK (second in xEdit). I've included few pictures that explains it well, look them up in the examples folder.
But as a quick example, when you look for Assault Carbine's EditorID - it will be WeapNVAssaultCarbine.

What about FormID? What is it?! It's a hexadecimal identifier of the object, and is used by the game. FormID is the second column in GECK. Look up the pictures just like with EditorIDs.
On Assault Carbine's example = FormID is 0008F21E. Now we remove first 2 characters (they indicate place in load order), and all leading zeroes until first non-zero character (thankfully, game "ignores" them). Final FormID is 8F21E. That's the one to use.