Old mattress pads work for the inside of the quilt. I don't think I'd use old sheets unless they are still serviceable. Maybe some inexpensive fabric or something from your stash that you really can't see using for anything else. Try just some large pieces of fabric with some kind of batting inside. After one of those (try WOF by 60" or so) , try a panel quilt (like a baby quilt). Then you can cut up the pieces - use the 12 x 12" squares, and make a rag quilt. You can give it/them away, including the panel quilt. Maybe you've seen John Flynn on a quilting show making a weed whacker quilt from the leftovers from his demos at quilt shows.
Then you should be able to take on a full bed sized quilt.