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.

The Music Volume Mixer mod is optional but it's an amazing mod to have if you want to adjust a specific soundtrack's volume during a heist (including custom tracks).

It also gives a solution around those pesky black screen bugs whenever you change custom tracks on the briefing screen.
(If you plan on changing to custom soundtracks during briefing, I highly advise to first select a main track from PAYDAY 2 and ready up. While in game, you can then change tracks to whatever you like in Sound Options -> Volume Mixer!)