After the 2 or 3 pieces of string have spun around and wrapped up in each other, examine which way they have spun. Repeat step 4. for the other edge of the wing.
Air likes to go from areas of high pressure to low pressure.
Looking at the back of the airplane, the left wing tip vortex goes clockwise, and the right wing tip vortex goes counter-clockwise, because of the curling action mentioned above. These vortices are also the reason that birds fly in a V-formation.