Place each 21-inch-long board on your work surface. Measure 2 inches from each end of every board and make a pencil mark. Position the edge of a carpenter’s square on the long edge of one board at a mark. Trace the arm of the square over the board with a pencil to make a straight line. Repeat at both ends of each board.
Attach a drill bit, slightly smaller than the diameter of your screws, to the drill. Drill two holes, 2 inches apart, through the pencil lines of each board.
Place the 4-by-4 posts on your work surface. These are the four bench legs. Attach the Phillips bit to the drill. Move two drilled boards to the center of your work table. These are the two arm rests.
Stand one arm rest up on a long edge. Butt the end of one bench leg against the arm rest at either end where the holes are located to form an L. Insert screws through the holes and into the bench leg. Attach the remaining bench legs to the ends of both arm rests in the same manner, making two squared, U-shaped frames.
Lay one frame flat with its legs pointing toward you. Set one drilled board flat on the frame with each end resting on one leg. Align the top of the board with the bottom of the arm rest. Insert screws through the holes of the board and into the legs. Attach three more boards down the legs in the same manner. Repeat these steps with other leg and arm rest frame.
Turn both frames over. Measure 16 inches up from the bottom of each leg and make a pencil mark. Set one drilled board on the legs of one frame, aligned with the marks. Insert screws through the holes and into the legs. Repeat with the other frame. These are the seat cleats.
Place the 36-inch-long 2-by-4 boards on your work table. These are the seat boards. Measure 1 inch from each end of every board, make a mark and then a straight line using a carpenter’s square. Drill two holes through each line, centered and spaced 1 inch apart.
Stand each frame on its legs with the cleats facing in. Rest the ends of the seat boards on the cleats. Insert screws through the holes and into the cleats.