You need to have winrar- don't worry it is a free program and it's very eay to use.
Copy and paste yourmaps into somewhere safe som they don't get lost.
If you want to switch back to your regulr unmodde maps just cop/paste the maps you backed up and paste em back in the maps folder.
Go here, download it, and click extract to extract it to your dexktop (easier to find) then there should be a white fuile on your desktop right-click on it click copy. then go to your computer drive (:c,:d,:g whatever) go to program files, microsoft games, halo, maps and right click then click paste. If it asks you if you want to replace this map click yes.
SO basicly:
my computer/

/ program files/ microsoft games/ halo/ maps.
That help?