Snap Inhale can be tricky to perfect but its such an enjoyable trick which looks so cool its well worth the practice!!
It could take you a week or so to get it right but once you have you can just get better and more confident with it.
You may find a mirror will help you refine your technique.
To start take a fairly long slow drag, about 3seconds or so. Your mouth needs to be quite full of smoke. As you draw in the smoke slightly curl your tongue up to the roof of your mouth. As you are opening your mouth to inhale just let your tongue move a little bit. This should make a nice thick ball of smoke come out of your mouth. If you now inhale fairly sharply through your MOUTH ONLY you will just have done a great snap inhale!!
Good luck, I hope this helps!