This is a topic that always comes during the first level of my Digital Painting class.
Students often make the mistake of painting an eye as a flat white surface.
Remember that understanding your subject is the key to paint it accurately.
An eye is a sphere, full of blood vessels and covered by skin that curves around it.

- Find the right references (as I did for this one)

- Pick the right brushes

- Use the software efficiently (a simple round selection with the additional layers clipped to the initial one, to make sure that the paint remains inside the eye).

This one was done very quickly (10mn) and when I look at it, I easily see myself spending a full day on it. But ... nope, the point is made!

Cheers!
B.

Bonus:
A bunch of eyes rolling down the stairs (yay!)

(Copyright Sony/Deharme for Bloodborne)