I was pretty surprised there were no guides for this, admittedly I had to click around quite a bit to find what actually popped this.From the Main Menu:
1.) Go to Multiplayer > Custom Game
2.) Scroll down to click Game Mode
and select 343 Industries
on any Game Mode to Save As
a new mode
4.) Add (or remove) a character and press
This should save a copy of the game mode as your own and pop the cheevo.