Start painting the picture. Firstly we need to paint the whole picture with the basic colors, selecting the Brush Tool. Each color should be inserted on a new layer:



Start a more detailed picturing. The eyes will be the first in our list: Choose the Pen Tool with Fill Path command to insert on the picture the pupil of the eye and the iris:



Then mark out the iris with the Elliptical Marquee Tool and paint it by parts, applying the Gradient Tool, then introduce the shadows, applying the Brush Tool and blur them out, applying the Smudge Tool.



Introduce a small flare also in the middle of the iris: