I had written this for another thread, but hopefully this may be of use to you.
1.) Base coat the whites of the eyes in Delta Ceramcoat Sandstone. Now, there are several reasons why I use this color. This will appear to have a more white-esque tone when applied next to your flesh color. It is a warm dark cream tone that will appear more natural and not too stark. Now, the whites of the eyes are in actuality more of a grey tone, but on a model....the grey looks too cool and unnatural and dead. This warm color will help avoid the stark look to the eye and add the warm natural feel that helps breath life into your piece.
Once the eye is based in Sandstone, use a thin black or charcoal grey paint to draw out the shape of the iris.
2.) Now take your iris color and draw that in, leaving a fine thin outer rim of the black/grey.
3.) Take a darker color and rim out the iris and draw in varied striations towards the center of the iris.
4.) Draw in your pupil. Then take a lighter value of the iris color and draw in more striations near the pupil in an outward motion towards the outer iris. Mix up a warm fleshy mauve color and rim out the outer eye.
5.) Sometimes you might want to add forced light, you can do this by simply taking an off white/ivory color and add light spots onto the iris. Seal your work and add a gloss coat over the eyes and your are done.
