Place the single 20-inch by 24-inch piece of plywood on a work surface. Lay a bead of glue on the 20-inch length edges.
Attach the two 20-inch by 18-inch sections to the piece with the glue, matching up the 20-inch sides. Use the wood screws and drill, attaching at the corners first, then placing at least two other screws in between. The piece looks like a U at this point.
Lay a bead of glue along the 24-inch length and on both of the 18-inch lengths. Place one of the 25 1/2-inch by 18-inch pieces onto the glue and screw into place. Here again, start in the corners then fill in between them. Repeat this procedure for the other side.
Wipe off any excess wood glue. The box is complete and ready for use.