I soak my dark flannels in vinegar overnight just to set the color, then wash and dry them. Just made a new flannel baby quilt, it has been so long since I made one, forgot to sew my lines of stitching starting in the center and working my way out to the edge, ended up with a ripple effect. When I clipped it, that straightened out the imperfections. I also remember that I needed to sew the first line of stitching in one direction and the next in the opposite direction, just to keep the fabric from rippling. Perhaps this information will help someone else.