Measure the distance where the ceiling will sit to the floor -- or the height of the wall if you are installing a wall. Cut a 2-by-4 that is 2 inches longer than the distance you measured, allowing you to wedge the board between the floor and the ceiling or about 5 inches below the top of the wall.
Cut three lengths from a 1-by-4 board that measure 2 feet long. Place one of the 1-by-4s so it intersects the top of the 2-by-4, and then drive three nails through the two boards to join them together.
Nail the other two 1-by-4 boards to the ends of the 1-by-4 you already nailed to the 2-by-4. Drive one nail through each of the 1-by-4s to anchor them together. Swing the loose ends of the 1-by-4s so they sit over the 2-by-4, and then drive nails through the 1-by-4s and the 2-by-4 to join them together.