I'd be happy to answer your question, as it is very good and has never been asked before. Oh wait, none of those things are true. SEARCH you bastard! It's at the top of the page. Go read some tuts. Go to
xbox-scene it's the best XBOX mod related website around. All you need to apply mods is PPF-O-MATIC and Flashfxp. The tut that guy posted a link to kind of explained all this, but what you have to do is start playing HALO, load the map you want to edit, exit, connect to your xbox with flashfxp, go to the Z drive, save the appropriate cache file(cache003 is blood gulch) on your computer, open PPF-O-MATIC, apply the patch to the map, and put the map back where you got it using flashfxp. -Just a tip, PPF-O-MATIC tells you to look for an ISO file, but if you change the file type to all at the bottom of the open window you can just patch the .map files. I guess I answered your stupid question anyway.
I've been dazed and confused for so long it's not true...