Anytime I am cutting a shape on the bias I starch my fabric first. It makes a world of difference. I usually have 3-4 applications of starch. I spray the fabric lightly and roll it up and put it in a plastic bag like we used to do back in the day with sprinkling clothes before we ironed them. Once the starch has had time to absorb into the fibers of the fabric I iron it and then repeat the process a few more times. Yes, it does take some time but well worth it to reduce the headaches that bias edges present when they stretch.