Tropit, from the videos I've seen, it's kind of an anything-goes sort of thing. Some people sew the pieces directly to the backing, some people use some type of thin or lightweight fabric (can be from old clothing) as the middle layer of the quilt sandwich, some use old blankets (or towels for smaller projects), and others regular quilt batting. Seems the heavier/thicker the "filler" is, the harder it is to sew through. Mind you, I'm not speaking from experience. I haven't tried it yet.