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
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)
mods
Download the mod and open it using 7-Zip or WinRAR.
Extract the main folder to mods > Menu Backgrounds > Assets.