You make a mod using programs found on files.halomods.com(HMT, Spark edit, etc). You mod the maps in your MAPS folder(blah/Prog. Files/Microsoft Games/Halo/MAPS) Then when u finish ur modding, just open halo, clik multiplayer, create an online game, and ppl will eventually join it, and the mods will b there.
Well mother, life is like a box of chocolates, you never know what you're going to get. Your life on the other hand is like a box of active grenades!