You draw on your scallops on the border(I use my Frixion pen) . You sew your bias binding on following the line. After you are satisfied with how the binding is on, then you can cut the border away for the scallops and hand sew the binding to the back.
If you try to put your bias binding on already cut scallops, the bias edges of the scallops are going to stretch as you sew on the binding.