The HQ Circle Ruler is an inside circle making it easier to be accurate. 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 further instructions and design ideas.
There is a PDF design sheet for this product as well which can be found in this document -