Draw lines along grid edges so as to divide the grid up into regions. Every region should have two-way rotational symmetry, and should contain exactly one dot which is in its centre.
Click on a grid edge to add or remove a line. Right-click on a dot and drag the mouse to place an arrow in a grid square pointing to that dot, to indicate that you think that square must belong in the same region as that dot. Right-drag an existing arrow to move it, or drop it off the edge of the grid to remove it.