After downloading the mod, use a program such as 7zip or WinRAR to extract the main folder of the mod.
Drop that folder into the mods folder which is supposed to be in the game's installation folder (don't know where that is? click here)
Have SuperBLT and Beardlib installed in Payday.
Download the Mod Zip and extract the File, named "Portal OST, Songs To Heist By - The Part Where He Kills You".
Place the Mod File inside C:\Program Files (x86)\Steam\steamapps\common\PAYDAY 2\assets\modoverrides. If the modoverrides folder doesn't exist, create it.