Cut a rope or string to a length slightly longer than the desired radius of the circle you would like to describe with your compass. The radius is the distance from the circle's center to its outer edge, or half the diameter.
Push a pencil or tack through one end of the rope or string at the spot where you would like the center of your circle. This point is the anchor for your compass.
Push the tip of a pencil through the other end of the string. The distance from this pencil to the anchor is the radius of the circle. Measure the distance with a ruler if you desire specific measurements for your circle.
Press down on the anchoring pencil or tack and fully extend the rope or string with the drawing pencil.
Maintain pressure on the anchor and even tension on the string or rope as you rotate the drawing pencil around the anchor to describe an arc or circle.