After downloading the mod, use a program such as 7zip or WinRAR to extract the main folder of the mod.
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
Extract this mod into mod_overrides.
If you dont place into mod overrides and instead place into mods, you will likely have issues with the weapon loading up.
CAFCW: Extract into mod_overrides.
Recommended for use with other custom attachments.