Lay the two 54-inch boards on edge, parallel and 10 inches apart. Set the two 10-inch boards perpendicularly between them on edge so they are parallel and 51 inches apart. Screw through the 54-inch boards and into the ends of the 10-inch boards. Use eight screws for this task. You should have a 13-by-54-inch frame.
Position the 33-inch boards perpendicularly inside each corner of the frame so their ends are all flush with one side of the frame. Screw through the corners of the frame and into the 33-inch boards. Use 16 screws for this task. These are the legs to your keyboard stand.
Turn the stand upright so it is standing on its legs. Lay the plywood on top of the frame so all of the edges are flush. Screw the plywood to the frame using 16 screws spaced 8 1/2 inches apart.