Figure out how large the circle will be first. If you have specific dimensions you can measure your piece of rope or string so that it is equal to half of the diameter. If you just have an idea of how large the circle will be, mark the place where the center will be. Then draw out a length of string or rope that will extend from the center of the circle to the outside edge. Add about six inches of string or rope to this length. Now you can cut your compass string so that its length will be the same as your circle's radius plus about six inches to work with.
Attach the string to a thumb tack by winding it and pulling a slip knot around the shaft if you are using your string compass to draw a circle on fabric or cardboard. If you are using a thicker string than can be tied to a thumb tack, tie it to a large nail, tent peg or dowel. This forms the point of the string compass. If the surface on which you are drawing your circle is porous, use something with a point that will penetrate this material and hold the compass in place while you are drawing the circle. On the other hand, if the surface is too hard to penetrate, tie the string to a dowel or a broom stick. In this case, you will need a helper to hold the point in place while you are drawing the circle using a string compass.
Tie the other end of the string to your chalk, pencil or marker. Make sure that the string measures the length you decided to make the radius of your circle in the first step above. If the string is too short, loosen the knots that you made to hold either the point of the string compass or the marker and adjust the length until it is accurate. If the string is too long, tie another knot and continue tying knots until you have shortened the string to the right length.
Ask a helper to hold the point of your string compass exactly in the spot which will become the center of the circle unless the circle will be smaller than the span of your arms. Then you can hold the point in place with one hand at the center and draw the perimeter's edge with the marker using your other hand. Probably you will be making a larger circle so ask your helper to keep the point tightly connected to the center while you move out until the string is tight and fully extended. Keeping the compass string fully extended and tight like this, draw the outside edge of the circle by pulling the string around the center and drawing the line that it subscribes. A string compass is a simple trick that will help you whenever you need to make a large perfectly round circle.