Try using butcher paper instead of thin cardboard. You can iron it to the wrong side of fabric (cut it 1/4 inch smaller than your fabric circle), press your s/a over to the back and then "running stitch" around leaving a small opening to remove the paper almost at the end of your completed circle. You get a good edge. I run a pencil around inside to loosen the paper from fabric and then use tweezers to pull out the paper (throw it away!) Hope this is clear!