Continue filling in the subarea until it is filled.

CAUTION: This exercise places a lot of stress on your wrist (at least it did on mine) and can cause some discomfort. Please take frequent breaks and let your wrist muscles relax.

Zoom in very close to the subarea in which you are working. This will make it easier to see where you should place the lines.

Continue filling in the adjoining subareas until they are filled in.

Only extend lines outside of the square on the guidelines where they will meet up with and join lines on the opposite side as noted here with the green arrows.

Take an eye and wrist break.

Continue filling in the adjoining subareas until the entire square is filled in.

Take a long break :-)

NOTE: I got a little carried away. Your cells (the irregularly shaped areas created where the lines cross) do not have to be so small. You could easily get by with half this many.