A) Box parts: Cut a 38" of 1x10 for the bottom of the box. Cut two lengths of 1x6 the exact width of the 1x10 (this should be about 9.5") for the ends, and then two pieces 36.5" long for the sides. Then cut four 5.5" bits of 1x3 for corner-blocks.
B) Box Assembly: Pre-drill all scre-holes. Attach the corner blocks to the inside of the long sides of the box, flush with the top, bottom, and ends, screwing from the inside. Attach the shord ends of the box to the corner-peices (and thus the sides) screwing from the outside. Attach the long bottom of the box to the sides and ends, screwing from the bottom.
C) Leg parts: Cut two 14" peices off the 6' 1x12, leaving 44" remaining. Cut two peices of 1x3 to fit along the top and bottom edge of each these legs. thus, 4 peices about 11.25" long. clamp these in place, and cut four more vertical stiles to fit beween them, (thus, 4 pieces about 9" long)
D) Template: Using a compass set to a 1" radius curve, make the template shown on a 9.5 x 14" peice of cardboard. The actual placement of the curves isn't especially important, except that you must leave room for the bench-seat lid to open. So the top of the leg must come in at least 1" on each side, and 1" down.
E) Leg Assembly Trace the pattern on the leg-board, and on the assorted trim peices, and cut them out with a jig saw. (Cut only the outer tracings on the leg-board.) Then screw the leg-board to the trim, from the back/inside. Rasp with an aggressive rasp, or power-sand the assembled leg.
F) Final Assembly Screw the box to the legs with long screws, from the inside of the box. Then flip the entire assembly upside down, and center it on the 44" bench-seat. Attach either two or three regular utility hinges, or a piano hinge to the back of the bench, and the bottom of the seat.
Sand and paint to suit.