After downloading the mod, use a program such as 7zip or WinRAR to extract the main folder of the mod.
Move the main folder of the mod 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 for PAYDAY2 folder). Create the folder if necessary
Simply put the contents of "Excalibolg.zip" into your mod_overrides folder and tadaa, you got a cool spiked club now!
