I piece my backings to use up left over yardage if appropriate. When seaming my pieces together, I use the factory edges as a guide to keep my seam straight but I also sew with a smaller stitch lenghth and a 1/2 to 5/8 inch seam allowance. After seam is sewn, I trim off the the selvages and them press open. It is much easieer to longarm this way.