Most circle attachments adjust to make circles from 2" up to 10" diameter. When you begin sewing with the attachment, the fabric has no option but to stitch in a perfect circle around the pin. In this example, the spotted white fabric has been appliqued using a satin stitch. It is a perfect stitched circle. After the applique is complete, multiple circles of decorative stitches have been stitched around it, creating a beautiful stitch out of perfect circles.
Sometimes an attachment will fit a different machine. The Pfaff attachment above is mounted on a Janome machine. In the following pictures, I am using the Janome attachment on a Janome sewing machine.
Picture 2 shows the pin covered by the black cap.
Picture 3 shows how you might mark to ensure you begin all rounds at the same place (optional). It also shows a mark to indicate the stopping place for a final round (for a specific design size).
Picture 4 shows the beginning of a round.
Picture 5 shows the round in progress.
Picture 6 shows the last round has been completed.