Grab any deck of cards and split the deck in two
*The best way to improve is by practicing*
Cut the deck in two halfs and place each half in a hand. Hold the halfs with the thumb on one end and fingers at the other
Place the two halfs closly together with your thumbs closest together. Slowly release the cards with your thumbs and get the cards to fall together in an alternating pattern
Click this link for a video that can offer more help.