White frogs appear with every move of the cursor in the main menu.
So each press of up or down would make a frog appear and it will hop across the screen.
The trick then is to press up and down fast enough to get up to 21 frogs on screen at the same time, since many might have hopped off the screen already if you're too slow.
My suggestion is to use two fingers and alternate up and down really fast.
When the screen looks like it's filled up to 21 frogs, press a button to have them disappear.
If you did manage to splat 21 frogs, the trophy will pop!
If not, go back to main menu and try again. :)
One additional tip:
Any press of a button would make the frogs disappear but it would also activate the selection you're on, meaning you can end up starting a new game, continue, etc, making retrying this a little bit of a pain on the top main menu.
What I found is a better place is in the chapter select, if you have passed more than one chapter, and using circle button as the button press to "splat" the frogs.
At least there, the circle button would cancel back to the top main menu and you just have to re-enter the chapter select to try again.