Make sure to first have SuperBLT, and BeardLib installed. Click the links in the dependency list.

Optional video tutorial for installing SuperBLT, BeardLib, and mod_overrides mods: https://youtu.be/rin_sjSJIkg

1. After downloading the mod, use a program such as 7zip or WinRAR to extract the main folder of the mod.
2. 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
3. Move the main folder of the mod from step 1 to the folder you navigated to in step 2.

For CustomOST version, place the tracks of your choice in ...steamapps\common\PAYDAY 2\mods\CustomOSTTracks

But if you're not willing to risk your game crashing (more often than it usually does), place the BeardLib version folder into ...steamapps\common\PAYDAY 2\assets\mod_overrides instead. It is also important to have Music Volume Mixer for it to work.