Instructions
Insert the "Halo 3" disc into your Xbox 360 disc tray.
Go to "My Xbox" and press "A" after the console recognizes the disc. Wait for the game to load.
Press "Start" to open the main menu.
Select "Forge," followed by the map you want to modify.
Use the onscreen options to modify the map. For example, press "A" to select an existing object. You can move the object elsewhere or remove it altogether. Alternatively, press "X" to create a new object.