Instructions
Turn on "Halo 3" and launch Forge from the main game menu.
Select Sandbox as your Forge map and hit "Start Forge."
Hit "Up" on the directional pad to enter Edit Mode and press "X" to open the Object Palette.
Select the "Scenery" palette and scroll until you hit "Wall" or "Double Wall." Both are the same height, but the double wall is twice as wide.
Place the wall wherever you would like on the map. You can rotate the wall by holding the left trigger button and moving the control stick, or place it in its current form.
Open the object palette again and select another wall.
Place this wall directly on top of the first wall.
Continue placing walls until your barrier reaches the desired height.