Place your hand palm up and rest the pen across the base of your middle and index fingers. The back end of the pen should be just under your thumb, and your ring finger and pinkie should be over the pen.
Apply pressure with your middle finger and release the pen from your thumb. This causes the pen to start spinning.
Bend your middle finger as the pen spins and allow the tip of the pen to pass behind the index finger. Your ring finger and pinkie should remain largely motionless and out of the way.
Allow the back of the pen to pass behind the middle finger and in front of the index finger. This is the difficult part that you'll need to really practice.
Complete the spin by catching the end of the pen with your thumb. As you finish the spin, you must straighten your bent middle finger and curl your index finger.