Place the chicken breast-side up, and run the center of the string under the neck in the front of the bird.
Bring the string up towards the wings and legs.
Use your thumbs to tuck the wings in as you bring the string around towards the legs. Keep the string tight to force the wings firmly against the body. The string should roughly follow the contours of the chicken breast.
Bring the string around between leg and breast, then give it one overhand knot and pull tight. The wings will be solidly pinned to the body, and the chicken breast will pop up. Note that this is NOT a solid knot - we just want to be able to tighten up on the string.
Bring the ends of string down between the chicken's legs, then cross the legs at the "ankles" above/behind the point of the chicken breast. Make sure your previous knot is still pulled tight.
Separate the strings, loop them around the outside of the chicken ankles, then tie a square knot to finish it off. The legs should now be cinched in close to the body.
Snip the extra ends of string, and discard them.