Try laying your small square ruler inside the circle and the one that is slightly smaller than the circle should work. I helped my daughter make her step daughter one, and I got the original pattern from
http://equilters.com/library/jeans/jeans_gallbaros.html
Hope this helps. We used lots of scrap fabrics, and used a varigated thread and a wide zigzag to hold the circle part down over the center fabric. We also put a few pictures we copied in a few of the centers. It turned out very nice. Good luck.