As properly as I know how to do it, anyway, I picked up on this by watching Smashified Art, and the artist David Harrington. Made in Gimp 2.
We shall start with an eye as you might do it.
NOTE: It appears flat. Now, I’ll show you how to make it look 3-D-ish.
First, you take a dark color, possibly darker than you’re comfortable with, and you
BLIND THE PERSON!!! cover the eye with the color. If you are doing a dark setting the color will be even darker, and you will not use white, you will use a light grey for the highlight.
Next, you take white, and smear that on top.
Next, we take the smudge tool, put the brush on hard, and then smudge the white DOWN into the grey.
Start pulling the grey back up and try mixing it a little bit. It takes some nitpicking, keep at it!
I added more white to the center, and more grey to the edge, because the center is the high point of the eye, right?
Smudge that, and then add the base color for your pupil.
Add a highlight to the top, and a shadow to the bottom, in the same directions as the light on the white of the eye.
Smudge that, and then try to make sure the turning point, the ‘corner’ if you will, of the highlight is near the center so that the pupil can cover it for a smoother transition. Mine is not directly in the center.
Add the pupil.
Get the smudge tool onto a soft setting, and then get the size so that it covers the pupil and part of the eye. All you have to do is click it a couple of times, and that will be a sufficient smudge so that the edge on the pupil is not so hard.
Add the light point.
And there we are, not necessarily a good eye SHAPE, I did this in like ten minutes but that is how you SHADE an eye. This applies to any eyeball, whether human or shaped like Sonic the Hedgehog’s.