Instructions
Travel to the outside of Ogrimmar. Summon your flying mount and fly to one of the guard towers along the wall.
Unmount on the ledge of the tower that has huge spikes along its edges. You'll see the windows of the tower have a sharp angle to that of the ledge.
Walk as far forward into the wall as you can go and then try summoning your mount. This may take a few tries, but eventually you'll glitch yourself through the tower and fall through the floor, underneath the city.