Make sure to first have SuperBLT, BeardLib, and WeaponLib installed. Click the links in the dependency list.

Optional video tutorial for installing SuperBLT, BeardLib, and mod_overrides mods: https://youtu.be/rin_sjSJIkg

1. After downloading the mod, use a program such as 7zip or WinRAR to extract the main folder of the mod.
2. Drop that main folder of the mod to assets/mod_overrides. The assets folder is supposed to be in your game's installation folder (don't know where that is? click here). Create the folder if necessary

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Weapon Painter: Extract into mods.
Weapon Painter is not technically required if you are okay with the basic gun. If you are looking for most of the blueprint parts, you need Weapon Painter.

CAFCW, MAT, etc: Not "required", but the XM4 was designed assuming you have these additional mods.

They all should be installable via extract into mod_overrides, but you can go on their mod pages for instructions.