Clear the board of flowers in Standard and Expert modes.
Clearing the board on standard is fairly easy, but the expert mode should prove to be more difficult. Basically, you will want to try and work towards the middle of the screen, as that sometimes makes it easier to focus on what you need to clear. It's also good to note that if one flower remains, then using the towel on it will also "clear" it and pop the achievement. If you aren't able to clear the board within the first three levels then you should restart as it will be almost impossible to clear the board in later levels. A good amount of luck factors into this as well, so just keep trying and you should eventually get it.