Position the shelf board flat on the working surface, with the 8-foot-by-3-inch surface facing downward. Align the rear board parallel to the shelf board and flush with the rear surface of the shelf board. Keep the 8-foot-by-1/2-inch surface of the rear board on the working surface and the 8-foot-by-2-inch surface flush with the rear of the shelf board so the two boards form an L shape.
Predrill 10 evenly spaced holes along the length of the boards, with the drill bit entering the rear of the rear board and passing through the rear surface of the shelf board. Fasten the boards together with 1 1/2-inch wood screws through the predrilled holes.
Locate your studs in the desired section of the wall by using the stud finder. Mark each side of each stud with the pencil. Hold the shelf against the wall in its eventual position with the help of an assistant. Position the carpenter's level on the top surface of the shelf, and move the shelf back and forth until it is completely level.
Predrill two holes in each stud, drilling through the rear shelf board and into the wall, while the assistant holds the shelf against the wall tightly. Make sure that the drill bit encounters the wood stud on each hole. Fasten the shelf to the wall with the 2 1/2-inch wood screws.