Have never used this method before, like to piece the back to make the back interesting, but when I started quilting this method was used. Most of the experienced quilters used plain muslin, it was available in wide widths. Cut the muslin in the length desired, two pieces. Remove the selvages and sew the two pieces together, face to face, lengthwise, both sides. What you have is a tube the length of the quilt, cut one layer of the tube down the middle. What you have is a large backing with seams on each side.