Things You'll Need
Instructions
Visit Bungie.net, the official website of Halo 3 developer Bungie.
Click the "Sign In" button on the site's upper right-hand corner.
Enter your email address and password in top and bottom fields to sign into to your Windows Live account. This account should be linked to your Xbox Live account.
Return to Bungie.net's homepage.
Hover over the "Games" tab on the website's top-left corner.
Select "Halo 3" from the drop-down menu.
Click on the "Online Stats &Files" button under the brief description of Halo 3.
Locate the clickable text that reads "Map Variants" near the middle of the page. Click on it.
Read details about custom maps by clicking on the map's title.
Click on "Download to Halo 3" from the map's details page.
Wait for the download to finish completely.
Access Halo 3 on your Xbox 360 console to use your new custom map.