If it isn't too big a project I make a paper pattern. I tape pages out of an old phone book together until they are the same length that I want to scallop. I then fold the paper in half, then quarter and then eighths depending on how big I want the scallops. I then unfold the paper and find a round object to trace that fits into the creased sections. If it works out perfectly then I use the paper pattern on my fabric. I find it easier to draw on the scallops to the finished tablerunner and sew the binding on before cutting the scallops. This way it gives you time and fabric to adjust if needed. I am sure others will have their own ideas. I have a scallop ruler on order myself to make this job easier.