1. Form an 'A' shape with your index and middle fingers.
2. Draw back your lips to cover your teeth.
3. Push the tip of your tongue back with your fingers.
4. Close your mouth around your fingers, forming a seal, and blow the hole between your middle fingers.
5. Practice, practice, practice. Eventually you can graduate to the one-handed technique to and rival that cool uncle of yours who could always hail a taxi or score a hot dog from 100 yards away.
For a more in-depth description and some helpful illustrations, scoot over to The Art of Manliness.