If you get into a lobby. Something will pop up on the screen saying 'Press right on the dpad for instant 70' or something along those lines. You do that, then leave the lobby. Prestiege, and the host should invite you back. You then follow those steps again. It shouldn't take no longer than 5 minutes.
Something also pops up saying 'Press left on the dpad for challenges'. Most lobbies have a progress bar now, so you will see the progress of all the challenges being unlocked. You should just do this on the last prestiege, else you'll lose them when you prestiege.