Step Four: Adding details

The eye consists of one large black dot, one small white dot in the center of that, and one small grey dot partially overlapping the white dot. Then I used the Airbrush to dab some dark grey around it to soften it.

The nose was drawn with the Paintbrush using dark grey "paint". Although you can't tell at this scale, there is also a thin line of light grey to give it some depth.