Instructions
Insert the second "Halo 3 ODST" disc, labeled "Multiplayer," into your Xbox 360 and start the Forge mode from the main menu.
Select a large map, such as Avalanche or High Ground, then select the "Start Game" option to begin playing Forge. Only large-scale maps allow Scorpion tanks to be placed in Forge.
Transform into the floating Forge Editor by pressing the "Up" button on the control pad. As the Forge Editor, you have the ability to fly to any point in the level to drop vehicles and other items.
Navigate to the "Vehicles" option in the Forge Editor menu and select "Scorpion." A Scorpion tank will drop in the exact spot you press the "A" button. You can reposition the tank by highlighting it with the Forge Editor cursor and pressing "A" to pick it up. Pressing "A" again drops the Scorpion tank.
Pause the game and select the "Save as New Map" option if you want to keep the tank placement configuration on the map for future games. Exit Forge by choosing "End Game" from the pause menu.