Step 14: The Ammo
Take the string, put a loop in it and push the free ends into a pea sized lump of Blu-tac.
Wrap it around a couple of times and then smoosh the Blu-tac into a ball.
The extra string inside will allow you some length experimentation to gain maximum distance when firing. A good length is about two thirds the distance from the hook to the axle along the arm.