Have a pen that is not too light or thin. It is also nice if the end of the pen is slightly heavier than the tip (however, only by a little). That being said, don't have too thick/heavy of a pen.
The grip. Lightly hold it between your thumb and your index and middle finger. Hold the pen near the tip (lighter end), as seen in photo.
Your middle finger is the one which does the most work - it will provide the force for the spin. Your thumb does not really move much.
As you can see, my thumb does not move much. My index finger just moves in the opposite direction of my middle finger so that it gives way for the path of the pen's spin. It comes back in to catch it
Remember: Don't push the pen too hard. You don't have to sit down and focus practice. Just do it subliminally in say, class, lectures or the office. You'll get it! 👍👌