Rotate the grid squares so that they all join up into a single connected network with no loops.
Left-click in a square to rotate it anticlockwise. Right-click to rotate it clockwise. Middle-click, or shift-left-click if you have no middle mouse button, to lock a square once you think it is correct (so you don't accidentally rotate it again); do the same again to unlock it if you change your mind.
Squares connected to the middle square are lit up. Aim to light up every square in the grid (not just the endpoint blobs).
When this gets too easy, select a 'wrapping' variant from the Type menu to enable grid lines to run off one edge of the playing area and come back on the opposite edge!