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.
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Woo, easy. Next, color in and shade the whites of the eyes.
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New layer underneath all of your current layers. Set the opacity to 70%.
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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.
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Under that layer, color the eye using whatever color you want. I'm using purple :3
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Merge the previous two layers. After that, take an airbrush and add some black to the top to give it a more natural blend.
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New layer! Set the blend mode to "Glow" and keep the layer at full opacity.
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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.
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Completely optional, but on that same Glow layer, I like to add some white highlights. :)
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Aaaand once you do that, you're all done!
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I hope this helped somebody! I'm not a very good teacher, but I tried!