Now we'll make layers for the whites of the eyes and for the pupils.

eye white

Select eraser→soft round brush (13 pixel). It's the initial brush, the one I used for the line art. As the name suggests, it will give you nice soft lines. Use this brush to draw lines around the areas you want to be white.

Now select eraser→hard round brush, and erase the area withing the lines drawn in step 1. Use the soft brush for areas where you're concerned about the shape, and the hard brush for the protruding eyelashes.

Create a white layer under the skin layer. Use "fill" (check "all layers"), then use a brush to fill the areas bordering the skin.

Now create a pupil layer above the white layer using the same process. (I used the fill tool a few times to cleanly fill the space.)

This is what our layers look like.

line art

skin

pupil