Lay a 30-inch board flat and horizontal. Tie your string around the pencil and draw an arc, starting at the bottom left corner of the board, that rises 2 inches at the 15-inch center of the board, and ends at the right bottom corner.
Draw another arc that parallels the first, starting 4 inches above the bottom left corner, and ends 4 inches above the right bottom corner. Cut along the lines with your jig saw, and repeat these steps, using your last 30-inch board. These are the bowed, front and back sides to your bow bench.
Position your bowed sides so they are parallel, 15 inches apart, and with their concave edges facing the ground. Position the 15-inch boards perpendicularly between them, so they are 28 inches apart. Drill 3/32-inch pilot holes through the bowed boards, and into the ends of the 15-inch boards. Drill two holes for each corner. Screw the boards together using the pilot holes. Drill pilot holes for every screwing application in this project.
Position a 17-inch board perpendicularly inside each corner of your bowed bench frame. Their ends should be flush with the edges of the 15-inch boards. These are the legs to your bench, and they must be even. Screw through the corners of the frame, and into the legs.
Position your bench upright so it is standing on its legs. Drill 10 pilot holes through each bowed board. Space the holes 3 inches apart, and ½-inch away from the top bowed edge of each 30-inch board. Start 1 and 1/2-inches away from the ends of the boards.
Position the dowels perpendicularly between the 30-inch boards, so their centers line up with the pilot holes. Screw through the pilot holes, and into the dowels.
Apply your wood putty over the screw heads and let it dry. Sand down the bench with the medium and fine grade sand paper, and wipe it down with your tack cloth. Apply your wood stain and let it dry according to instructions.