1. Tie a Knot
Fold the string in half, and in the center tie a small slip knot. It should tighten up when you pull on each end.
2. String it through
With the string ends in your hands, slip the string over the door frame. It should fit through the crack where the door meets the frame.
3. Lasso the Lock
Loop the slip knot over the lock, and pull tightly so that it secures itself around the lock.
4. Pop the Lock
Pull on the ends of the string until the lock pops up.
To see this trick in action, check out the video below: