Wrong section to post this in. Request move to Halo forge Discussion. But if you need advice, here's what i have. Make it on any map you want, and follow the rules of MLG, which I'm not certain of so you'll have to find that out for yourself. There's a lot of rules you have to follow for MLG maps, and forge skills neccesary to create a good looking map with a good MLG layout. Go to the Forging 101 and MLG website and do some research on all that MLG stuff, and eventually you'll know how MLG maps work and how you want to create yours. Good luck.
You'll want to create the map on Foundry or Sandbox, as they have the most objects to create an original and unique MLG map on.