I have never built one but have observed the styles and have designed a wall bed, which is different. I think you must choose the style, either wood and cushions, or all upholstery. Wood and cushion sofas are simple in the working mechanics and materials. "All upholstery" sofas involves special mechanical folding hardware which you need to decide on first, then the upholstery work is often elaborate. A google search for "folding sofa bed" brings up lots of variations. The simple ones slide out rather than fold out.