Instructions
Play a game of "Halo 3" in the environment you wish to record for a screenshot. This is best done by starting a match in Forge mode by yourself so you can concentrate on playing and setting up a good shot.
Move your character to prime locations and face the scenery from many angles. Maneuver your Spartan to desirable sections of the map or press "Up" on the directional pad to enter the editing mode in Forge for more freedom. As the editor, you can fly to any point in the level and take interesting first-person views of the scenery.
End the game and enter Theater mode from the main menu. Select the last game you played and replay the recording.
Press "X" to bring up the snapshot toolbar. Press "Right" and "Left" triggers to fast forward and rewind the recording until you find the moment you want to save as a screenshot. Rotate the point of view with the "Right Thumbstick." Hover over the "Take Screenshot" button and press "A" to take the picture.
Choose to save or discard the image. If you save the shot, you will have to name the image.