Haha, I'm surprised .this has not been submitted considering it is the most used glitch. What you do is either A)the easy way or B)the hard way.

A) build a brick wall, walk towards it, while against it and walking against it, double tap up pretty quick. there you go. Haha, it's harder done than said.

B) find an uneven spot on the ground, double jump off of it. or easier, shoot the ground diagonally with your rifle, then double jump off that with option a, double tapping up. don't give me bullshit of this not working, just challenge me in game and ask, i'll show you.