The first thing you wanna do before you try and get it over the index finger, is to make sure that the index finger is completely extended. This will enable a smooth transition when you try and pivot the packet above your index finger.
After you've done that, you press downwards using your pinky and ring finger (though more so with your ring finger, your pinky is there at the back of the deck mainly for support). You will notice the packet will pop up just slightly. At this point, you start curling your index finger back and the packet should slide nicely over the nail of your index finger and back onto the deck.
I know that this whole thing sounds like a lot of steps but in reality when you're actually performing the move it shouldn't be more than a split second once you get it up to speed.
Hope this helps.
-Daren