Detailed instructions for a classic combination
Cut the 2 by 8 into three 41-inch planks for the seat. Set the planks in place on the seat supports so that the back edge of the seat is flush with the outside of the posts.
Leave a slight gap between the planks for drainage. Attach the planks with 3-inch screws. Round the corners with a jigsaw, then use a router with a ½-inch roundover bit to round off the edges.