Step 1: OK! First things first - color in the eye area.  You should probably use a general midtone of the desired color of the eye. In this case, meh boi Judah has really blue eyes! ^^

NB. Ignore the other layers in the figurative images - in my real WIP picture there are LOTS more layers.  I just combined them to make them less distracting in this tutorial.

Step 2: Now that we have that easy step out of the way, create a new layer, set its layer blending mode to multiply, and paint in the darker areas of the eye.    We'll add more depth to this in a little bit.  For the darker color I chose a dark greyish purple color, but make sure you have at least 2 colors for this darker part.  Note my lighter purple as well.

Step 3: OK, I forgot to screen-capture this step, but select a feathered brush and set it to pen pressure (sorry to non-tablet users - you can use the fade option, but it takes more work), and soften certain areas of the eye.   I usually only soften up the edges at the top where it fades into the dark... but I don't soften it too much.   X_X  sorry I didn't screencap it...  :(

 

Step 4: Um ^^ and here you can see where the options to feather your brush are located.  Make sure that shape dynamics are off, and keep this brush this way for a while.  I usually use a pretty big brush (because this picture is 300dpi resolution) so in this case I think my brush as size 35.    WhOO I'm rambling ;_;   again I'm sorry. MAKE SURE after you have softened the upper area of the eye that you preserve transparency, as it works magical wonders!  Oo.    So, in this step you can take advantage of that lighter color and add a little bit of dynamic coloring to the eye with variations in the tones.  I also added a slightly darker color to the top, as you may see...  Oo.

Step 5: Create a new layer, just above the shadow layer, and make sure its blending mode is set to linear dodge. Any more layers you make for the eye will be in a linear dodge mode.  Choose a highlight color a little darker than you really want - it will turn out lighter with linear dodge in effect.   Using your feathered brush from before, add some highlights along the lower part of the eye.
 

Step 6: After that, create another linear dodge layer, and move it above the line art (because that stubborn line art likes to cut across places you don't want it!).  On this layer you should pick a color that isn't quite white.  Notice that the color I have is a very pale robin egg blue.  Use that color and a small brush, preferably one that will get you sharp edges.   For mouse users, just use one that isn't feathered. Tablet users should use a small feathered brush with shape dynamics.  Now draw your highlights wherever you want - just try to make them pretty and follow your light source (i.e. make your biggest shine where the light hits the character).  When you are done with this step, duplicate the layer.

Step 7: Almost done!   On the duplicated layer, go to Filter > Blur > Gaussian Blur. I blurred mine at a radius of 18.  When you finish that you should have a prettyful eye!   XD     OMG, I TOLD EVERYONE I WAS BAD AT TUTORIALS!

Um... I hope this helped someone anyway!

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The finished picture (with eye :).