Determine where the hidden balls are in the box, by observing the behaviour of light beams fired into the box from the sides.
Click in a square around the edge of the box to send a beam into the box. Possible results are 'H' (the beam hit a ball dead-on and stopped), 'R' (the beam was either reflected back the way it came or there was a ball just to one side of its entry point) or a number appearing in two squares (indicating that the beam entered one of those squares and emerged from the other).
Click in the middle of the box to place your guessed ball positions. When you have placed enough, a green button will appear in the top left; click that to indicate that you think you have the answer. You can also right-click to mark squares as definitely known.