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
mods
Must Have SuperBLT and BeardLib to work
BeardLib is important, because it has a library for the music
Once you have both, put the custom song in # mod_overrides folder