Assemble the frame by laying it out on your work area for visual planning. Place the two shorter sections of 2-by-4s on each of the outer ends. Align the two long sections on either side, and add the other long section to the back.
Screw a side long section to the end of a 4-by-4 piece using the wood screws after predrilling the holes. Make sure it overhangs on the outside end by the width of a 2-by-4. Do the same thing to the other end. Finish the side by screwing on the other long section about 6 inches from the first in the same manner of overlapping the 4-by-4. Use two wood screws on each intersection side, angled from each other.
Repeat for the other side.
Screw the end sections into the 4-by-4s without any overlap, bringing the sides together using the wood screws in predrilled holes.
Insert the seat supports 12 inches apart and screw in from the sides, keeping the top flush.
Lay the planks on top of the frame and screw it into place, countersinking the screws in predrilled holes. Use a little wood putty to fill the holes.
Sand the bench thoroughly until it is smooth. Wipe it down with a dust rag and stain if desired. Cover with a polyurethane finish and allow drying for at least 24 hours.