I use Sketchbook Pro, and I dunno if this tutorial will work for other programs, but if it does yaaaaayyy.

So first things first, prepare the eye.

Woo, easy. Next, color in and shade the whites of the eyes.

New layer underneath all of your current layers. Set the opacity to 70%.

On that layer, take a black inking brush and shade the top. Use a smaller brush to make horizontal lines in whatever pattern you choose. I tend to keep mine pretty simple, though.

Under that layer, color the eye using whatever color you want. I'm using purple :3

Merge the previous two layers. After that, take an airbrush and add some black to the top to give it a more natural blend.

New layer! Set the blend mode to "Glow" and keep the layer at full opacity.

On that layer, take your original color for the eye(in my case, purple) and use an inking brush to add two diagonal strokes through the whole thing.

Completely optional, but on that same Glow layer, I like to add some white highlights. :)

Aaaand once you do that, you're all done!

I hope this helped somebody! I'm not a very good teacher, but I tried!

~Aly