Make sure to first have SuperBLT, and CustomOST 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 mods\CustomOSTTracks. The mods folder is supposed to be in your game's installation folder (don't know where that is? click here).
3. Move the main folder of the mod from step 1 to the folder you navigated to in step 2.

As for mods overrides tracks, either just slap the whole folder in your assets folder in Payday 2's directory (if you don't have a mods_overrides folder of your own already), or simply pick out the tracks you're interested in and put them in your pre-existing mods_overrides folder.