This is actually quite simple, just hard to describe in words. Check out the pictures and photonotes for details.
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. (You could turn it into a square knot
if you need to, but I just keep tension on it during this next part.)
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.