After downloading the mod, use a program such as 7zip or WinRAR to extract the main folder of the mod.
Navigate 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
Move the main folder of the mod from step 1 to the folder you navigated to in step 2.
Extract this into mod_overrides.
Weapon Painter: Needed for some of the blueprint variants.
Extract Weapon Painter into mods.
CAFCW: Needed for support in adding grips to some custom weapons.
Extract CAFCW into mod_overrides.