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.

Install SuperBLT & CustomOST
Once done, download & unzip this mod, then drop the 'Mystic Castle' or the 'Hang Castle/(Inverted)' folder into:
\PAYDAY 2\mods\CustomOSTTracks