1. You can use your thumb to slide the card forward a bit before using your index finger to pull it down. Once you've mastered the grip and the strength needed from your index, you can then stop using your thumb.
2. Make sure the card is fully flat and upside down before trying to "twirl" it with your index and middle fingers.
3. When you twirl the card, give your wrist a little spin. It makes the move looks so much better and more impressive.
4. Slow down, you don't need to rush the moves. Do one move at a time and master each step first. With enough practice, speed will just come naturally.