Stand the two 1-by-18-by-16-inch boards parallel to each other with the ends flat against a work surface. Space the outside edges of the boards 60 inches apart. These boards serve as the bench's legs.
Apply a bead of wood glue across the top end of each leg.
Lay the 1-inch-by-18-inch-by-60-inch board flat on the top ends of the legs. This board serves as the bench's seat.
Drive four evenly spaced wood screw through the top of the seat into the top ends of the legs using a screw gun. Wipe off any glue that oozes out from the joints with a damp rag.
Apply a bead of wood glue to the ends and one edge of the 1-inch-by-8-inches-by-58-1/2-inches board. This board serves as the center support. Only one center support board is required because it is placed in the center of the seat and not only will support the seat but simultaneously strengthen the legs.
Insert the center support between the legs so that the edge with the bead of glue is flush against the center of the bottom of the seat. The ends of the center support board should be flush against the insides of the legs. One-inch dimensional boards measure an actual thickness of 3/4 inches, so the 58-1/2-inch-long center support board should fit perfectly between the legs' inside edges.
Drive three evenly spaced wood screws through the outsides of the legs into the ends of the center support board. Wipe off any glue that squeezes out with a damp rag. Allow the glue to dry for 24 hours.
Sand the bench with 120-grit sandpaper. Wipe off the dust after sanding with a tack cloth.
Apply an even coat of wood finish with a paintbrush in the same direction as the wood's grain. Allow the finish to dry for four hours. Apply a second coat of finish and allow it to dry for 72 hours before using the bench.