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.
Weapon Painter: Extract into mods.
eyeAim Camera: Extract into mods.
CAFCW: Extract into mod_overrides.
Weapon Painter is technical optional if you are okay with the SIG Desert color, but is needed for the FDE and Black texture variants of the base parts.
eyeAim Camera: Technically optional for the motion sick, but recommended for some extra impact and weight on the reload animations.
CAFCW is highly recommended if you want to load up your weapon with various accessories, like the Tacticool (x) Packs or the Tactical Vertical Grips.