The main cantilevered deck of the treehouse is designed with a broad sweeping curve at the front, so the outmost deck joist will have to be curved to match. How to make curved lumber, in this case a curved 2 x 6? Fairly easily, if you start with decent sized scrap plywood, best thickness around 1/4" to 3/8" or so. Set up a table saw with the rip fence set at the width of the lumber you are trying to match, in this case 5-1/2 inches for the nominal 6" lumber. After ripping, this is what you end up with. The length really is not critical, as long as about half are at least six to eight feet or longer.