Place two 3/4-by-18-by-32-inch plywood panels parallel to each other on a work surface with the 3/4-by-32-inch sides resting flat. Space the panels so that their inside edges are 16 1/2 inches apart. Have a helper hold the panels in place. These are the top and bottom panels of the bench.
Position one 3/4-by-16 1/2-by-18-inch plywood panel between each end of the top and bottom panels, one at a time. These are the side panels of the bench. The 3/4-by-16 1/2-inch edges should be flat against the work surface.
Drive trim head wood screws through the top and bottom panels into the 3/4-by-18-inch edges of the sides using a screw gun. Space the screws every 2 to 4 inches. You should now have a rectangular box assembly. You helper can stop holding the panels once you have fastened them together with the screws.
Center the remaining 3/4-by-18-by-32-inch panel over the open side of the rectangular assembly that is facing up. Align the top, bottom and side edges so that they are flush with the outside edges of the assembly. This is the back panel of the bench. Drive trim head wood screws through the back panel into the edges of the sides, top and bottom. Space the screws every 2 to 4 inches.
Stand the assembly upright so that the opening in the assembly is facing you. Apply an even coat of wood finish onto the entire bench assembly with a paintbrush. Allow the finish to dry for at least four hours and apply a second coat. Wait 24 hours for the second coat to dry before using the bench.