The HQ Big Swiss Cheese Ruler has inside circles in three sizes. It's easier to be accurate with these inside shapes. Reference lines on the outside help with alignment of the ruler with seam lines and previously stitched circles.
To get your foot into the inside of the circle line up the notch with your foot and needle (needle in the up postion) and you'll find you can easily put it inside to start stitching.
See the Video below for design ideas and instructions for using the ruler.
There is a PDF design sheet for this product as well which can be found in this document -