Lay your two 24-inch boards on edge, parallel and 21 inches apart. Set two 21-inch boards perpendicularly between them so they are also 21 inches apart. Screw through the 24-inch boards and into the 21-inch boards. Use eight screws for this task. You should have a 24-by-24-inch frame with flush corners. The measurements might seem off, however standard lumber is specified by its nominal size (2-by-4-inches). The actual size is really 1 1/2-by-3 1/2-inches.
Position a 30-inch board perpendicularly inside each corner of the frame. These are the legs to your turntable stand. The ends of these boards should all be flush with one side of the frame. Screw through the frame and into each of the legs. Use eight screws for this task.
Set your turntable stand on its legs so the frame is above the ground and held up by the legs. Screw a 21-inch board perpendicularly to a pair of legs. The board should be six inches below the 24-inch frame and its ends are flush with the sides of the 30-inch board. Use four screws for this task. Screw another 21-inch board to the other pair of legs in the exact manner. These two boards hold the plywood for the DVD shelf.
Screw the last two 21-inch boards to the legs so there is a 21-inch board beneath and parallel with the two 21-inch boards in the last step. They should be 16 inches below the two 21-inch boards in the last step. Use eight screws for this step.
Lay your 24-inch plywood on top of the stand so all of the edges are flush. Screw the plywood to the stand using eight screws.
Lay the 21-inch sheets of plywood on the 21-inch boards so its edges are flush with the ends of the 21-inch boards. Screw the plywood sheets to the boards using four screws for each sheet.