Plan the Circle Size
Place a stake in the location selected for the center of the circle; if you are lining on grass, a metal tent stake works well, and if on a hard surface, any heavy object with a vertical peg is sufficient. Use a tape measure to prepare a string with the length of the radius of your circle. Loosely tie one end of the string around the stake, and confirm that the string rotates freely.
Create a Guide for Your Circle
Hold the end of the string, and walk away from the stake until the string becomes tight. Walk around the circle, maintaining the tightness of the string, and lay down a second piece of string along that path. If you are marking on a hard surface, tie a piece of chalk to the loose end of the string attached to the peg, and draw a line as you walk around the circle. Confirm the accuracy of your work by measuring the radius of the circle created by your string or chalk line in several places.
Paint Your Circle
Prepare your line striper with paint. Lifting up the back two wheels, which, in combination with the front two wheels, normally keep the striper in a straight line, pull the paint lever and carefully walk around the circle, tracing the string or chalk line you created. Your walking speed will determine the heaviness of the paint, so walk slowly for thick, long-lasting lines.
Peter Schuffelen: Surveying (Laying Out Soccer Fields)