Open the menu at the top left corner and select "game options". There is a setting called "Show Available Tiles". Turn that on and complete a puzzle. Easiest one would be the "Turtle" puzzle, which is the first puzzle in the easy difficulty.
You can either beat it legit or spam the hints for every move by pressing "H" on your keyboard and then clicking on one of the glowing tiles. Repeat until the puzzle is completed.
Done deal, and you have a shiny new achievement!