Create a square with the felt piece by folding in half widthwise and lengthwise. Use a tape measure to determine the waist measurement and divide that number by pi (3.14). This will be the radius to cut your top circle on the square of felt.
Place the top of your tape measure in the upper left corner of your folded square of felt. Draw a line for the measurement you got previously. Continue measuring in a circle and tracing the radius measurement around until you get to the other fold. This will form your waist circle.