The key point here is balance. When I do it, the card is just resting on my middle finger, and the thumb practically feels like it isn't touching the card (you could slip a Rizla paper in the gap, that's about it!)
It's a bit like spinning a basketball on your index finger... I can't personally do that, but people who can have just tried and tried and tried and eventually found the sweet spot.
Most card backs have a visual clue to the centre of the card. Aim at the centre, spin gently, and just keep trying. I always found with anything like this that sitting watching a film or something on TV is a good time to practice a move like this, because it seems to lessen the tedium
(The tedium of the TV or the repetition of the learning?—Hmmmm... depends what I'm watching!)