This is where I always run into trouble. You have to assume or know that the edges are straight on the sides. I have so much trouble getting the fabric squared up in the first place. I have read everything on here, watched all kinds of tutorials but when I get done it will be just a little off. You have to fold it straight line it up straight hold the ruler straight,cut straight. Then when I pick it up somewhere in that process I messed up. I can do it if someone squares the pc first for me.