After downloading the mod, use a program such as 7zip or WinRAR to extract the main folder of the mod.
Drop that folder into the mods folder which is supposed to be in the game's installation folder (don't know where that is? click here)
FrenchyAU's tacticool attachment packs are STRONGLY recommended, because all of them have new unique stats with GIlza. They are ordered in order of importance/impact on weapon modding.