Push left thumbstick (crouch) to give yourself a quick thrust upward. Push it repeatedly to rise higher / faster. It's easier to take off if you jump and then crouch while you are rising. Once you're in the air, just the occasional well-timed crouch will keep you there. With this jetpack method, you have full ability to use grenades and guns while flying around, and normal running around and jumping are unaffected. It takes a little bit of practice to get used to it.
I recommend setting the camera to 3P, turn off the HUD, give yourself infinite ammo and grenades, and see how far you get. Note that I swapped the jetpack effect with an attached shield effect, so for some reason, if your shields go all the way down, your jetpack will no longer work. It's a downside of using a biped attachment, not hard to work around, but that's for later.
Also note that there's something in the campaign mode that automatically kills you if you fall for a certain time, regardless of whether you touch the ground or aren't near a deathzone. So if you zoom to the top of the map, be sure to slow your descent periodically. Kind of like the bends.
Feel free to use this in your own mods, but always give credit where it is due.
