We are doing this project right now. My suggestion is to get a sailrite catalogue and they are offering a free CD this month on tips.
My wife has done projects on the boat like a new bimini, dodger and windows as well as curtains and sail repairs. My suggestion is that you get new "reflex foam" for the cushions. as it drains moisture easily. We originally were going to get fabric in one of the discount stores, but were instead use a sunbrella furniture fabric...yachtsman series which is soft as well as resists mildew and moisture. Originally we were going to do a mix of poly and cotton, but due to the ever present moisture a boat has switched over to 100% acrylic. Get good zippers also. Feel free to PM me as we are just getting started with our project.