Instructions
Hold the pen in a vertical position and rest the the pen's center of gravity on your middle finger. The center of gravity is the point where the pen doesn't dip down on either side. Hold the pen with your thumb and index finger. The grip you will use is similar to the normal way you hold the pen when you write.
Push the pen upwards with the middle finger as you fold the middle finger in towards the palm. Straighten your index finger at the same time.
Rotate the pen around your thumb. The key is keep the thumb immobile. Catch the pen with the index finger as you complete the rotation.
Practice the trick several times every day for a week. At first, spinning the pen feels awkward but faithfully practicing the trick eventually makes it easier and more natural.