Which version should I use?

Try using the MCM version first, as it makes it changing settings more comfortable.
If you have many mods and you're having conflicts with ModLib and MCM, use the Simple version.

The game crashes at start

Probably having to do with blocked DLLs. Read how to fix that in this troubleshooting guide.

When multiple mods use ModLib and MCM the game may crash. When updating mods they may leave unused files behind that may crash the game.

MCM standalone is a requirement for this mod. If you still have problems, use the Simple version, which doesn't depend on anything and should just run fine.

What are the configuration files for?

If you're using the MCM version, the in-game menu handles most of the configuration you can tweak, but some settings can't be put there so you have to use the configuration files. They are always optional features and you're not required to edit them before playing.

If you're using the Simple version, all the configurations for this mod are in the ModuleData folder.

The configuration files are very easy to edit. Text editors like Notepad++, Sublime or Atom make it very easy because they color the different parts, but you can also do it with the regular Windows Notepad.

Mod is installed, but there are no banners

The default settings are tuned to be balanced and rewarding as your game progresses. If you're starting a new game it's not likely you'll see any banners. As soon as your army starts to grow and get experience you'll start seeing more and more banners. You can of course tweak the configuration files to your liking.

Archers, crossbowmen and horse archers

Any unit that carries a bow or a crossbow in battle will lose that weapon whenever they're given a banner. This is necessary, otherwise they drop the banner. They still use their melee weapons. Whether they get a banner or not is configurable, of course.

Also, if told to Charge, the units with banners will rush the enemy, leaving their fellow archers behind, so be careful. Order them to Advance and they will stay together.

Why do some units drop their banners?

Units drop their banners at the start of a battle when they have a two-handed weapon that can't be used one-handed.

Additionally, when a troop falls off a horse or is in low health, they will drop their banner and equip their shield if they have it. There's also a chance they will drop it when retreating (by default, that chance is tied to their experience). This is all configurable.

The mod BannerPeasants, created by Apastron152e9, adds a banner to most if not every troop type by modifying the troop trees, and has a Python script that will programatically add banners to custom troop trees with a lot of options, which is a great tool for other modders. You can see what rules this script follows to add banners and replace equipment in that mod's description. Originally, we both used the same core idea and uploaded on the same day with a few hours difference completely independent of each other!

I later developed a different way to achieve this, which is the dynamic assignment you see today.

Bear my Banner equips units with banners during the battle loading scene instead of modifying the native equipment sets. This makes it highly configurable and compatible with any mod that changes or adds troop trees. There are also many other features that introduce banner variety, units behaviour and ease of use.