Cut all lumber to your needed specifications. Sand all faces of your lumber with coarse grit sandpaper.
Gather one 24-inch by 24-inch sheet of plywood and two beams. Lay the beams on opposite edges of the sheet, parallel with their outside edges flush with the outside edges of the sheet. Glue them in place. Reinforce the glue with three wood screws per beam, evenly spaced and driven through the sheet into the beam beneath.
Repeat step two with the remaining 24 by 24 sheet and the other two beams.
Connect the two sheets by attaching a 48-inch by 24-inch lumber sheet to each, forming a C-shape. The 24-inch ends of the long sheet should overlap the 2 by 4 beams to align with the edges of the 24 by 24 sheets. Glue in place, then reinforce three wood screws per beam.
Turn the C-shape into a rectangular frame by attaching a second 48-inch sheet opposite the first. Glue and reinforce just as you did the first sheet.
Lay a third 48-inch sheet on top of the frame. Glue in place, then reinforce with one screw per corner, driven through the sheet and into the end of beam beneath. Further reinforce with a line of finishing nails along each edge, driven through the face of this sheet and into the edge of the sheet below. Use three nails evenly spaced along each short side and six nails evenly spaced along each long side.
Flip the box over so it rests on the third sheet. Lay the final 48-inch lumber sheet over the final open face. Attach it to the box with your three hinges. Place one hinge each 4 inches from either end of one long side. Place the third hinge at the center of the same long side. Screw in place using the included hardware.
Apply painter's putty over the heads of all nails and screws. Wipe away excess. Wait for putty and glue to dry.
Sand the bench with fine-grit paper, smoothing the wood and removing excess glue and putty.
Apply one coat of outdoor stain to all surfaces, interior and exterior. Allow to dry. Apply a second coat to all exterior surfaces.