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 CustomOST version:
Download the file called "Hades CMM CustomOST ver.rar"
Open the file
Drag and drop the file on (Payday2/mods/CustomOSTTracks)
Open the game.
Once in the game, go to settings -> sound -> custom heist playlist. Scroll to the bottom and check the music mod.