I would approach it in segments. As in a set of surfaces. The top seat section (where you put you butt) I would just deal with the ribs then deal with the sides and bottom. same with the back rest. I would use sweep as my main tool to make these separate "sheets" then I would use blend to tie the parts together and then join. sort of like you have separate sections of leather that you sew together. the seam lines would be my guides.