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 from step 1 to the Maps folder. Create the folder if necessary; BeardLib should create it automatically when launching the game with it.
How to Uninstall this piece of shit:
Navigate to your PAYDAY 2 directory and find "Maps"
FInd this map, its called "roomtest"
Delete it, put it in recycle bin and empty it maybe idk