In the Main Room, go to the stairs.

Walk up as much as you can and turn around, and if you look up a bit, you will see a little ledge on the wall.

If you run and jump high enough you should land on the ledge.

If you are in the glitch right, when the Zombies come up the stairs and try to get you, they will not be able to touch you.

This works better if you have a flame thrower as it has unlimited ammo and takes a few tries to get into the glitch.

And because of this you don't want to come out of it in case you cannot get back into it.

Have Fun.