|binding on left is cupped, on right is flat|
First of all, you need a little tool. It doesn't have to be just like mine, but it does need to be something firm enough to hold down your binding and thin enough to slip under the presser foot if needed. I use a little wooden "spork" that I kept after getting a tasting sample in the grocery store. I also have one I made out of a craft stick. I suspect an old credit card with a notch cut in it might work, but I haven't tried that.
Here is the tip. As you sew around the curve, use the tool to hold the seam line flat as you allow the outside of the binding to slightly ruffle. This puts some "ease" into your binding and will allow it to easily turn over the raw edges without cupping. (If you are VERY new to this, you will notice that the binding is folded in half and the raw edges are aligned with the raw edges of the quilt. Then when you turn it over the raw edge, you have a fold to sew down.)
That's all there is to it! Easy, unless you were never told to do this! I hope you find this helpful. Sometimes those of us who have more experience at these things forget to mention these little tricks. I have given away several quilts with cupped scallops. I tried ironing them into submission, but that only works temporarily.
Happy quilting or sewing, and until next time......