Your bench needs legs, which in this case are two trapezoids, each of which include four separate pieces of those freshly cut 2x4s. The legs are the two 17-inch boards connected at the top by the 20-inch boards and at the center by 23.75-inch boards.
Connect the ends of the 20-inch boards to the 17-inch boards with wood glue, followed by screws. Then connect the 23.75-inch boards 4 inches (10 centimeters) below. Do this twice and you should have two A-shaped legs that will form the ends of the bench.
Next grab the 31-inch boards, which will be used as back supports. These boards should be screwed and glued to the lower horizontal slat of each A-shape that you just created and should intersect at the top corner of the A. The angle of the intersection is up to you; just make sure that it's the same on each side.
Now you can connect the two A's together with the 40-inch 2x4 (the seat stretcher) by gluing and screwing it to both the bottom horizontal boards of the A and the bottom ends of the back supports where they connect with the horizontal boards.
It's now time (already!) for the final step. All that hard work so far would pretty much be for naught if no one can actually sit on the bench. So, you'd better lay the seat.