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
Drop into Witcher 3 Main Menu folder which is besides the music one into menubackgrounds\assets . This mod is required for the background. For music, drop into mod_overrides.