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.
(ignore step 3 because i changed the folder structure so it should find it's way)
4. Move the folder of the mod to steamapps > common > PAYDAY 2 (don't know where to find it? click HERE)