Stand the two 2-by-12-by-16-inch boards upright on a work surface, parallel to each other and 56 inches apart. These are the legs.
Lay the 2-by-12-by-60-inch board on top of the legs, with its ends flush with the top of the leg boards. This is the seat.
Drive four evenly spaced flush-trim wood screws through the top of the seat into the end of each of the legs using a screw gun.
Turn the bench over so the legs stick up, and center the 2-by-4-by-56-inch board between the legs with the 2-inch side against the underside of the seat. This is the center support.
Drive two evenly spaced flush trim wood screws through the outside of the legs into each end of the center support.
Apply wood glue to one end of enough wood plugs to fill each screw hole. Insert a plug into each screw hole with the glue end in. Allow the glue to dry.
Sand the wood plugs flush with the surface of the wood using a fine-grit sanding block.
Apply a clear coat finish to the bench. Allow the finish to dry.