Flannel makes great backing. I always wash and dry the flannel several times as it usually shrinks a lot. I then heavely starch, before pin basting. This seems to help giving a smooth back. Also if you are straight stitching it helps to use a walking foot. When free motion quilting be careful to move the sandwich gently, not pull it. I have spray basted before pining. good luck, flannel makes a warm cuddly backing.