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
All you need to do is place it into your mod_overrides folder. If you do not have a mod_overrides folder in your assets folder, create one just named mod_overrides.
Having difficulty? Consult this guide: https://imgur.com/a/m5xwKmP