Put one end of ruler in center of circle. Trace edges of ruler outside circle, drawing two parallel lines the length of your pot's height. (For instance, if your pot is 4 inches tall, the lines you trace outside the circle should be 4 inches long—and 4 inches past the circumference of the circle.) Repeat on opposite side, then perpendicular to the lines you created. When you finish, you will have what looks like a circle with a cross shape behind it.