I got a scallop ruler from Joannes a few years ago and used it once. I was in a gadget buying mood... Anyway, if you use a scallop edge on your quilts alot, this would be handy. However, if you just wanted to try it once, you could:
1. get a long piece of cardboard and draw a straight line across it
2. mark two sides of a lunch plate depending on how deep you want the scallops
3. Line up the marks on the plate with the line on the cardboard and trace.
4. Cut out and use as your template.