Someone requested I do one of these, so here it is! I'll explain the steps better in text below.
Step 1. Draw an eye shape and an iris. For cartoony styles like mine you can mess around with different shapes to see what looks good for your character. Also, color in the white of the eye on a layer below the lines.
Step 2. Create a new layer above the eyewhites but under the lines. Color in the iris with a base color-- usually dark.
Step 3. Using lock alpha on the color layer, create a gradient with lighter colors. I used the airbrush tool. I find it looks best if the curve of the gradient follows the curve of the iris.
Step 4. Create a new layer and set it to "multiply" instead of "normal". Using the airbrush tool, and a darker color that is more grey than the others, lightly cover the iris. It should still be darker on top and lighter on the bottom.
Step 5. Create a new layer and set it to "overlay" instead of "normal". Using white, create a light area that follows the curve of the iris and ends slightly more than halfway up it. Then I use the soft eraser to blend the edges a little, but that isn't necessary.
Step 6. Create a final layer for the highlights. Depending on the light source in your drawing these will move around, but I like to do one circle and one slash.
Feel free to try it out! And if you have any questions you can ask me. I know I don't always explain things well and I don't bite, I promise! c: