First practise this motion. This spreads the cards.
Take half of the deck and hold in this position.
Slide your thumb down from the top edge to the bottom left corner.
Then move your index finger to the bottom edge.
Finally spread with your thumb using the motion in step 2. Takes a bit of practise to get the right pressure on the cards.
Once you've learnt to do it in either hand. You can do two at the same time. :)