Find the letters from A to Z scattered and hidden on the picture of Santa Claus by moving your brush with your mouse on the letter then left click it. You can see all the letters you have not found on the bottom section of the screen. It will turn to white each time a corresponded letter found.
A context menu should be shown when right-clicking or, on touch devices, long-pressing in the game. This menu especially allows to play in full screen and control the volume.