Measure out a 36-inch long curve, 2 inches deep, on the piece of hardboard.
Cut out the curve and smooth out any rough spots with the sandpaper.
Use the hardboard curve to trace out a curve on the 36-by-7 inch board as close to the edge as possible. Cut along the curve and treat the piece as scrap.
Measure in 2 inches from the last curve and trace and cut another curve. Repeat this step once.
Take the two pieces you just created and sand smooth any rough areas from the cut.
Lay the 18 inch boards on-edge, 12 1/2 inches apart and lay two 12 1/2-inch boards flat, running between them. Attach one 12 1/2-inch board flush with the top edges of the 18-inch boards, and one 5 inches from the bottom.
Cut a 3/4-by-2 inch rectangle out of two corners of the 14-inch square board, so that the rectangles run parallel to each another.
Attach a 12 1/2-inch board to the bottom of the edge between the two cut-outs you made, which will be the back edge of the seat, so that it is perpendicular to the 14-inch square board.
Attach the assembly from step one below the front edge of the seat, with the flush 12 1/2-inch board at the top and facing forward.
Attach the 10-inch boards to the open bottom edges of the seat, running from the front legs to the cut-outs.
Attach the curved pieces, curving outward, in the cut-outs, securing them to the seat and the boards running beneath the seat, such that the ends sticking out as the back legs are even with the front legs.
Attach a 12 1/2-inch board between the back legs, 5 inches from the bottom.
Attach the remaining four 12-1/2 inch boards between the curved boards at the top, forming the back of the chair. Attach them starting 1 1/2 inches from the top, with 1 1/2 inches between, as near to flush as possible with the front of the curve.
Sand smooth and paint or varnish.