How to hold the string. By winding it through your fingers in a serpentine fashion, you can easily regulate the string's tension by holding your fingers closer or farther apart.
Start at the top left corner of the form. With one hand, hold a length of string there before the edge, and gently start to wrap the form with the other hand. For the first go around, go easy on the tension--think sharks swimming around their prey. You don't want to pie the form because of your impatience, do you?
When you come around to the starting corner again, see that the new loop overlaps the starter length at that corner. That way your string won't unravel from the beginning.
This is the tricky but fun part. When you get to the next corner from your starting corner, Hold the string tight in your hand, and using your thumb as a fulcrum, pull the string tight around the corner. Maintain tension and wrap on.
Remember to overlap your string at the starting corner.
When you've made enough loops to satisfy your paranoia, bring your string to the second corner (the one where you tightened the string with your thumb, remember?) and hold the string against the form in such a way that your string lies above the other loops.
With your makeup rule, or tweezers, or pocket knife, push the string through the other loops to the bottom. This is easiest if you pierce the string at some point.
With your tool of choice, hold the loop down and pull taut on the end of the string, seeing that the "knot" lies as close to the corner as possible (where most of the tension is). Cut off the offending extra length of string and...
Behold your beautiful tied-up form!
Looking towards China Beach and the California Palace of The Legion of Honor.