Instructions
Place the "Halo 3" game disc into the Xbox 360's disc tray. The game will run. Allow the game to fully load. The main menu will appear. Select "Forge." You will be redirected to the "Forge Lobby." Select the option labeled "Map." A list of maps will appear. Select one, then select "Start Forge." The game will appear.
Press the "Up" arrow on the Xbox controller's directional pad. You will enter "Edit Mode." Use the controller's analog sticks and the left trigger to navigate through the map. Locate a replaceable object. These objects will appear green in "Edit Mode."
Approach the object and press the "Y" button. The object will disappear from view. Take a look at the "Budget" meter located in the bottom right corner of the screen. It will have increased. Each time an object is deleted, a player will make money and the "Budget" meter will increase.
Once you are satisfied with the map you've created in "Forge," press the "Start" button, then select "Save Map As." The map will be saved to your multiplayer map database.