Step 1: Pen Choice
You can choose any pen you'd like for spinning; they all work the same. However, I highly recommend you do not use wooden pencils. They're a real pain in the butt because of their even weight distribution.
Throughout this Instructable I will be using the infamous Pilot G2 because I believe they are easiest for beginners.
Step 2: Grip
This instructable is for right handed folks.
You will start by finding the balance point of you pen. Do this by laying your hand down on a flat surface and lifting one of your fingers. Wherever the pen balances is where its balance point is. (Duh).
Put your middle finger on the balance point, and your thumb slightly to the right of it on the opposite side. Allow your finger to fall into its own place. This is the grip you'll use!
Step 3: The Spin
The true trick to the spin is that your thumb NEVER MOVES. All of the spinning is initiated by your middle finger as it pushes against your thumb. Again, the thumb never moves. You are not squeezing the pen, you are using your middle finger to push the pen around your thumb. Your index finger is there to catch it when it comes back around. Once the spin has begun, you will move your middle finger down and catch the pen in a writing position.
You won't get it on your first try. Just keep practicing! I found it helpful to do half spins until I could master a full spin.
Step 4: Tricks N' Stuff
It's considerably harder to spin your pen in the air as I am doing here. (Obviously, I dropped it at the end...) Practicing on a hard surface is definitely noteworthy.
Step 5: Practice!
Now that you know how, you can do it with only a little practice!