Edson,
As mentioned, the large scales really let you add alot of detail. Many great methods were offered and here's just another one (method that is, not necessarily great), you'll see alot of the same info with some additions;
1. Paint eyeball offwhite (leaning towards grey, not cream-look at your eye in the mirror)
2. Paint a think brown line under the upper eyelid (burnt umber, etc)
3. Paint the iris black
4. Paint a dark shade of your eye color (dk brown or grey blue) in a star/sunburst pattern
5. Lighten the eyecolor mix and paint the upper 2/3'rds of the iris from the direction of your light source
6. paint a highlight mix of your eye color in the upper 1/3rd of the irs again, from the direction of your light source
7. If blue. paint a thin "starburst" of lt brown in the center of the iris (as the images provided by Stephan)
8. Add pupil (remember if outdoors to make them smaller
9. Paint a reddish brown/pinkish line on the inside of the lower eyelid (terra rosa+white)
10. Add catchlight "dot" of white to the very highest points of each eye from the direction of your lightsource.
I hope that makes sense. The only difference I think I added compared to the other good advice was the highlighting and shading of the iris. It reduces the chances of getting the "mad stare". Also remember to position only about 2/3rds of the iris as visible. Make sure the upper eyelid covers anywhere from 1/4 to 1/3 of the iris/ If you paint a perfectly round iris it has a tendancy to look psychotic.