Combine a stone on the bottom (brought by the goldfish) with the same stone on the playing field.
Click on a stone on the bottom and then on the same stone on the playing field. You can also discard a stone (top right). Use the crown stones to get extra help.
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.