Place the two pieces of 22-by-84-by-¾-inch plywood (ends) on a worktable. Measure from one end and make marks at 3, 18, 57 and 70 inches. Place the framing square on each of the marks and draw lines across the width of the plywood. Mark the bottom side of the lines for the shelves.
Place the five pieces of 22-by-46½ by ¾-inch plywood (shelves) on the worktable. Apply glue to the edges, place them between the ends and secure them with the finish nails.
Apply glue to the edges of the home theater cabinet. Place the ¼-inch plywood on it and secure it to the cabinet with the 1d box nails. Be certain that the good side of the plywood is facing inside the theater cabinet.
Turn the home theater cabinet over. Place the 2-by-84-inch lumber on the front of the cabinet. Next, place the four pieces of 3-inch lumber between the two 84-inch pieces as indicated in the illustration.
Apply glue to the ends of the 3-inch pieces and clamp them tightly with the bar clamps. Clean excess glue with a damp cloth. Let them dry over night.
Remove the clamps and drill pilot holes through the frame with the 1/16-inch drill bit. Apply glue to the edges of the cabinet, place the frame back on it and secure it with the finish nails. Sand the joints with a belt sander and 100-grit sandpaper.