well if you mean how to play the map eg downloaded map called testmod2000 you extract the map using winzip winrar 7zip ect then if halo is on you turn it off then you open your map directory C:\program files\microsoft games\halo CE\ there look for the map folder and drag and drop the .map file to that folder and then start up halo. you can add maps while halo is on nothing wrong is going to happen your just not going to be able to play the new maps till you restart halo hope this helps