The FingerFlip is a fun trick. It is also harder than it looks. When I started practicing freestyle again about 2 years ago, this was the hardest of my old tricks to get back.
I recommend that you practice this move stationary, probably on carpet, to help you learn the proper timing for the jump and the flip. This will make it much easier to learn from a roll.
Ride forward at a moderate speed. Make sure you are rolling smoothly.
Put one foot on the tail, and the other in the middle of the board, or maybe a little further back. You should be on the balls of your feet.
At the same time, reach down and grab the nose with your lead hand, thumb on top.
Now the tricky part. You must jump up, while still grabbing the nose.
I find that if I try to keep my knees close together, and I face slightly forward, I get a much better jump, and the trick looks better too.
Once your weight is off the board, use your hand to flip the board. You must flip it hard.
Pull your knees toward your chest.
As the board flips, you want to be in the air right over it. I find that if I start the move facing slightly forward, rather than sideways, I stay over the board much better. If I'm facing sideways, I jump sideways, away from the board.
Many people start trying to flip the board before it is in the air. This trick is all timing. If you still have your weight on the board when you try to flip it, you will hurt your fingers. Get in the air, then flip it. Keep your feet pulled up long enough to give the board a chance to flip. Beginners will often lower their feet too early, landing on an upside-down board. In frame 6, the board has come around, and I'm lowering my feet to catch it.
In frame 7, I smack my feet down, catching the board, and in frame 8 I land it and roll away.