Lay two of the longer boards flat and parallel side-by-side. Make the ends flush. Apply wood glue along the seam between the boards. Press the boards together with a cinder block on the outside edges. Allow the glue to dry.
Place the remaining two long boards parallel, narrow sides facing you, with the ends flush. Set a distance of 12 inches between them.
Place a short board at each end of the upright long boards with the ends of the long boards butting against the sides of the short boards. Make the ends flush. Apply wood glue along the seams created at the corners and allow the glue to dry. Use a pair of cinder blocks against the short sides to hold the frame together.
Hammer a pair of nails through the sides of the short boards into the ends of the long boards. Repeat this step for each corner to sturdy the frame.
Measure and mark the distance between the interior sides of the long boards on the remaining short board. Cut the board to size. Fit the board between the long boards in the center of the frame. Glue the seams made by the placement and allow the wood to dry.
Apply a bead of glue around the edge of the bench top. Set the frame on the glue. Make the ends flush. Press down on the frame on all four sides so the glue gets a good grip. Let the glue dry.
Flip the bench top over so the frame is underneath it. Hammer four nails through the top into the frame on each side. Make the nails equidistant from each other. Flip the bench top over so the frame is on top.
Position the end of a post along an inside corner of the frame. Drill two screws through each side of the frame into the post. There should be four screws holding the post to the frame. Repeat this step for the remaining three posts.
Flip the bench over to stand on the legs. Paint or stain the bench to your needs.