The glitch is simple. Set up a game with BG Extreme. Then, click the left arrow twice and hit F5 at the same time. An error may or not pop up. If it does, simply hit ok and cancel the starting of the game. Next, click room settings, DON'T change anything, and hit ok. There you go, match is set-up.
However, I do believe that you WILL NOT get extreme rewards with this glitch. I think you use to, but Aeria may have patched it or something. I'm not 100% sure. You can keep trying it though haha.