Kill off innocents and friendlies in Arizona until two of the areas send out a radio distress call to the Ranger Citadel. I personally used Ag Center (after saving a bunch of people in the questline) and Rail Nomads Camp, and just went around shooting them all (easiest to hit the right trigger to bring up the attack options, then Y to filter to friendlies, then just RB to the next one and pop 'em off.)
After you kill enough innocents in an area, there's a radio call where Ranger Citadel checks in to see what you're up to. Move on to another area and, when you've done enough killing of innocents there, another radio call goes out naming you sworn enemies of the Desert Rangers and that you should be shot on sight.
I wouldn't recommend doing this on a save that you intend to keep for your main playthrough unless you like making things tougher.