To do this glitch you have to go to the right window to the back and break/shoot the 2 bottom pieces of the window.

Then rebuild it and jump on the first board, then the second board and then jump again and you should be standing on the door.

Then stay there and make sure you bought a gun at the beginning of the level, you can shoot the zombies all you want, but don't worry if the zombie comes beneath you.

You can't shoot it or you run out of ammo because they will kill themselves if you wait a while.