First, I did NOT make this tutorial, it comes from the sight you goto!
1) First, goto halomaps.org
2) Click on Halo CE maps at the top.
3) Find a map to your liking.
4) ( this is the more complicated part ) IMPORTANT! First I must emphasize that all the files and maps contained on this website is for the Halo Custom Edition Game and NOT the standard Halo for PC game.
The Halo Custom Edition game (Halo CE) is a free unsupported upgrade and standalone program that will install alongside of your Halo PC game. Unlike Halo PC, the Halo CE game allows you to play and or create custom maps for multiplayer on-line play.
If you have installed the Halo CE game using the standard Windows installer then your installation directory is "C:\Program Files\Microsoft Games\Halo Custom Edition" There should be a \maps folder under that directory. This is where the map files go.
Full Path: "C:\Program Files\Microsoft Games\Halo Custom Edition\maps"
To install a map file first be sure that Halo CE is NOT running. Using an archive program like WinZip or WinRaR or open the *.zip file and extract the *.map file and place it in your \maps folder : "C:\Program Files\Microsoft Games\Halo Custom Edition\maps".
Start Halo CE and go through the process of starting a LAN Server. Verify that the new map you installed is available.
5) ENJOY!! Remember step 4 is what they wrote. Not what i wrote!!!~~ Thank you!