how to create a new halo4 game type???

i’v tried everything on my xbox 360 game halo 4 to create a new game type on halo 4 but it wont work i anyone knows how please message me.

Choose a gametype(any gametype) and change w/e you want then save it as a new one

EDIT: Go to Custom Games select Game Options change w/e you want and press Y and it should ask you if you want to save it as a new gametype.