Instructions
Turn the Xbox 360 on and load the Halo 3 video game. Press the "Start" button on the main home screen to open up sub-menus. Select the "Player" button to open up the "Player Selection" screen.
Scroll to the player to activate auto-aim. Select the player with the "A" button. Press the "A" button once more to open player options. Scroll down to "Player Settings" and press the "A" button.
Scroll down to "Auto Aim" on the menu and use the "A" button to turn the function on. Press the "B" button three times to return to the main menu of the Halo game.