get this first:
a BIG leaf

(1) make a circle with one hand
Put your thumb over the fingernail of your index finger. Be sure to leave a hole in the center. Place the leaf over this hand.

(2) cup your other hand and smack the leaf
With your other hand make a cup, like you'd use to hold water, but upsidedown. Smack the leaf HARD!

(3) POP!
If done correctly, you'll make a loud pop. This is because air is forced through the leaf. Your hands will probably hurt after doing this a few times, too.