make sure you wash and dry the flannel twice first...it will usually shrink a lot and it will pull the denim out of shape!! You can sew a really narrow zig zag around the whole outside of the quilt a 1/2 inch in and the flannel and the denim will rag. I might even go around it twice, to make sure the seam stays good. I would use a good 1/2 in seam to construct the quilt and a very narrow zig zag to seam the quilt with, too. The zig zag stitch will be stronger and help prevent the flannel from fraying out in the seam line.