How do you create your own game type in Halo Reach?

first you have to go into Custom Games and choose a game type from the type that you want (eg. Slayer or Juggernaut), so If you wanted to make a Slayer game type, just choose, like, Team Slayer or something. Then when it goes back to the main custom games menu press "X", then just go through all the different stuff and change what you want.