A very easy way to do this is find a map (A good one is level 4-1) with at least one frog on it. Then if you very carefully move as close as possible to the frog it will begin to lick without being able to hit you. If you get hit dont worry, just adjust your butterfly back a little bit and let go of the screen as long as the butterfly is far enough away but yet close enough for the frog to try to lick you it will count. It will look something like this:
EX: (Frog = F, Tongue = ~, Your Butterfly = B
F~~B