Lay the 20 inch by 24 inch plywood section on a flat surface. Apply wood glue along the two 20-inch edges.
Press one 20 inch wide by 18 inch tall section of plywood along one glued edge, so the wood is standing 18 inches tall. Drive one 3 inch nail through the plywood edge every 6 inches to join the two sections.
Press the 20 inch wide by 18 inch tall section of plywood along one glued edge in the same standing manner as the first piece. Drive one 3 inch nail through the plywood edge every 6 inches to join the two sections.
Apply wood glue along the 24 inch edges of the base plywood, as well as all four side 18 inch edges. Press one 25 1/2 inch by 18 inch section on to each side of the base.
Drive one nail every 6 inches along the perimeters of the two 25 1/2 inch by 18 inch wood sections to join all five wood pieces together.