I have reupholstered a whole living room set and there are patterns available at craft stores and wherever patterns are sold. Good luck, it is a big undertaking!
Take pictures as fabric is carefully removed and separated, number the pieces and use as patterns, use pictures to help put back together.
It is quite a task but not impossible. I suggest going on line and searching youtube.com for tutorials. By watching them, you'll learn if you are up for the job of reupholstering your sofa. Good luck.