Place the 24 1/2-inch piece (will be the leg) on a worktable. Measure from one end and make marks at 9 1/4, 12 1/4 and 15 1/4 inches. Set the carpenter's square on the marks and draw lines across the width of the leg. Make a mark at 1 1/2 inches from both edges on the 9 1/4- and 15 1/4-inch lines.
Set the compass on 2 inches. Put the point of the compass on the 12 1/4-inch mark as close to one edges as possible and draw a semicircle. Repeat this on the opposite edge, then set the compass to 1 1/2 inches.
Put the point of the compass to the 1 1/2-inch marks on the other two lines and draw arcs from the edge of the leg to the semicircle. Repeat this process on the opposite edge of the leg. Cut the lines with a band saw and sand the cut edges.
Drill three pilot holes 3/8 inch from the 12-inch edge of the 14-inch lumber (top and bottom) with the 1/16-inch drill bit. Drill one in the center and one 3 inches either side of the center. Next, drill 3/8-inch countersink holes on top of the pilot holes. Keep the holes no deeper than 3/8-inches.
Apply glue to the ends of the leg and secure it to the top and bottom pieces with the drywall screws. Clean up the excess glue with the cloth. Secure the shelf brackets in the corners created by the leg and the top and bottom according to the manufacturer's instructions.