Instructions
Turn on your Xbox 360 console and wait for the dashboard to load up.
Press up on your controller until you reach "Game Marketplace" and select "Explore Game Content."
Press up again on your 360 controller until you reach "Titles A-Z." Move the the letter "H" and press "A" on your controller.
Find Halo 2 from the list of games and click on it with the "A" button.
Click on the maps you want. Select the "Download" option to save to your console.