Instructions
Set your class up with a small machine gun and the secondary weapon of your choice. You will also want a "Tactical Insertion," as this allows you to come back to an area once you have completed the glitch.
Choose the "One Man Army Perk" so you can switch classes on the fly. This allows you to get a new Tactical Insertion each time you successfully complete a step in the glitch. Your other two perks should be "Lightweight" for better jumping and "Commando Pro," which protects you from long falls.
Align yourself with a small overhanging ledge such as a roof or billboard. This ledge must be just below your standing height, but above your height when crouched.
Push your body up against the ledge and look down. Rotate until the ledge is slightly slanted in your view. A very small amount of slant is necessary for the glitch to work.
Pause the game and change your control settings to "Legacy Southpaw."
Crouch underneath the overhang and strafe a few steps to the left. Hit the stand button on your controller. If you stand up, you need to move further under the overhang.
Move slightly to the right and try to stand. You are looking for the exact point in the game geometry between being able to stand and not being able to stand. When you hit this point, you will be shot into the air.
Drop a Tactical Insertion as soon as you reach the top height of the map. If you are killed, you will appear at the Tactical Insertion without the need to perform the glitch again.
Stay above the roof or building below you. If you pass outside of its boundaries, you will fall.