From Halopedia, the Halo wiki
Place a single box in front of one of the windows, then a crate on top of that. Move the crate as close to the window as you can. Now place a Machine Gun Turret as close to the window as possible, having it face away from the window and into the room.
Exit Edit Mode and man the turret. Aim as far upward as you can, so that the camera (which is now in third-person view) goes behind the window. Now enter Edit Mode again. You should appear outside of the map.
Once outside, it is possible to place Teleporters, Respawn Points, vehicles, and even Scenery outside of the map. There are two kill barriers outside of the map. If you touch the pipes (or the space above them), or if you walk past the corner of the level, you will instantaneously drop dead. Amusing deaths can result from placing a Warthog so that the passenger seat intersects a kill barrier, and then trying to climb into it. If you try to throw a standard grenade into the map while outside, it will get stuck and shake. It will then disappear; if you were near the grenade when it disappears, it will deplete your shields.