|You'll need two things, and you probably already have them...a thumbtack and some masking tape.
Whatever width [or diameter] size circle you want, you'll use half that measurement [the radius]. Measure out to the left of your machine's needle that much. Lay your thumbtack on its head [pointy end up], so the point is at the correct measurement, and tape it down.
Find the center of your fabric, and slip it onto the thumbtack. Smooth the fabric under your presser foot and sew...you won't even have to guide it. Just make sure the fabric stays on its pivot point, the thumbtack, by putting your finger on it.
I'd love to give credit to the clever person who thought of this, but I don't know who it was.