After downloading the mod, use a program such as 7zip or WinRAR to extract the main folder of the mod.
Move the 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 for PAYDAY2 folder). Create the folder if necessary
For the optional version, it can work on it's own just like the original mod but you will not get the physics that Keeru?!'s mod brings