Theres a tutorial on halo-mods, but the easiest way for me when im at friends houses. Just download HCE (halo cache editor) then put the ppf files you download from halomods.com and put them in Ntsc/Ppf folder and then it will show up. Now, go to hce on your xbox, press back, then clear cache (will load up halo), now load up the map that the hacked file is used for. Then just load it up, quit out, wait till you get to the main menu of halo. Press the reset usually (back, start, right and left trigger), , now go to hce, press back, go to patch map, click the hack you want, now click the map your using it for. It will patch, when it says complete PRESS A, then B, then start (will load halo) now click the hacked map.
P.S. Don't load more then 1 map when doing this. You need to clear cache each time.