Place the 1-by-12-by-13-inch boards parallel to each other with the outside sides 32 inches apart and the 1-by-12-inch ends flat against a work surface. These are the legs.
Position the 12-by-32-inch side of the 1-by-12-by-32-inch board across the top 1-by-12-inch ends of the legs. This is the seat. The ends of the seat should be flush with the outside sides of the legs.
Drive four evenly spaced flush-trim wood screws down through the top of the seat at each end into the ends of the legs with a screw gun.
Insert one 1-by-3-by-30 1/4-inch board between the legs on each side with the 1-by-30 1/4-inch side flush against the underside of the bench seat. These are the cross supports.
Drive two evenly spaced screws through the outside of the legs into the ends of the cross supports at each joint.
Position the 1-by-12-by-30 1/4-inch board between the legs and up 3 inches from the bottom ends of the legs. This the the lower shelf board.
Hold the lower shelf in place and drive four evenly spaced screws through the outside sides of the legs into the 1-by-12-inch ends of the lower shelf board at each end.
Sand the bench until the wood surface is smooth using a fine-grit sanding block. Gently round any of the sharp corner edges on the bench with the sanding block.
Paint the bench with a wood finish of your choice. Allow the finish to dry for at least 72 hours before use.