1. Rename the mod to "bloodgulch.map" if it hasn't already been done.
2. Halo Demo > Maps > bloodgulch.map, drag it to a different location.
3. Drag in the mod.
4. Simply join a server with the mod compatible or make your own server. It will automatically show.
Tips:
It needs to be named "bloodgulch.map" EXACTLY.
Don't get confused!
Make an "Originals" folder for the original bloodgulch.map so if there's an accident it'll be okay.