Find two marbles and an empty location lying together along a line. (that is "marble-marble-empty" or "empty -marble-marble"). This pattern can be both in horizontal or vertical direction.
Select the marble away from the empty location. Then select the empty location. The selected marble will jump over the adjacent one to the empty location (now the pattern will be "empty-empty-marble" or "marble-empty-empty"). The middle marble gets removed by this. Similarly, go on removing the marbles one by one till you reach a state when there are no two marbles together with an adjacent vacant position.