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 into your mod_overrides.
CAFCW: Needed for other attachment support such as custom optics.
The M4 Extras: Adds a bunch of blueprint parts.
Not strictly required if you are happy with the base weapon, but highly recommended