After downloading the mod, use a program such as 7zip or WinRAR to extract the main folder of the mod.
Navigate to mods\CustomOSTTracks. The mods folder is supposed to be in your game's installation folder (don't know where that is? click here).
Move the main folder of the mod from step 1 to the folder you navigated to in step 2.
how to install
1.unzip the .zip file
2.put the mod file in the your payday 2 custom ost traks folder (your steam folder location\Steam\steamapps\common\PAYDAY 2\mods\CustomOSTTracks)
3.done!