EDIT: Use the Anti-Alias option in the Shape3D plugin.
Put a check mark on it, Set to level 3 ~ 4 would be enough.
This will make your "3d" object look much much Smoother!
Use this option for the last step as selecting it will make rendering much slower
==================================================================
Easiest eye tut as of post date 
This tut aims to teach you there are different ways to "skin" a 3d object.
And how Shape3D "wraps" the "skin" for spheres.
And to make eyes like:
Plugins you will need:
Shape3D
Panelling Effect
Gradient Bars
Ok, Lets go!! Start with 800x600 image
1. Make 2nd color Alpha=0
Click on 1st color again.
(We will only need to change the 1st color for the whole tut, Keep 2nd color's Alpha at 0)
Use Black color :LinearGradient: as shown in image below.
(I have highlighted the end points with blue color and direction with red arrow for the whole tut!!)
Note: Hold the Shift key when using Gradient so you get a nice Horizontal gradient!!
Note2: Don't forget to use "Ctrl+D or Enter key" to "deselect /end" the gradient session.
so when you select the next color, you gradient color won't change with it.
2. Change 1st color to the eye color you want.
(I have used yellow so you can see clearly)
Use :LinearGradient: as shown in image below.
3. Change 1st color to black again.
Use :LinearGradient: as shown in image below.
4. Add new layer
Use Gradient Bars (Top, S=20, S=0)
5. Use Panelling (V=-22, H=0, S=1)
(Or Panel down until you can see a 1 pixel line of the eye color, as shown in image below)
Gaussian Blur (P=2)
Flatten image
6. Use Shape3D.
You can use the setting I used, or adjust away 
7. Add new layer
Change 1st color to white.
Use 
as in image below.
Flatten image, and you are done!! 
Hope this helped you to learn about how Shape3D works.
If you think about it, you can do much more then just eyes with this method!!
Have fun & show me what you made!! 8)
EDIT: Some more examples for you to see 
======================Extra======================
Very simple steps the get the Slit pupil that everyone is asking about.
After you finished drawing the eye.
Add new layer. Draw a triangle like image below. (only need to draw on 1 side)
Then Dup layer, goto Layers--->Flip horizontal, then merge layer down.
You should have something like image below.
Use Shape3D (full sphere map)
And you will see the slit pupil already there, (assuming you still have the setting from when you made the eye)
You will need to adjust the X-axis maybe a little of the Y-axis to Fit the pupil onto the eye.
Done
===============================================
Comments & Questions welcome. 8)
Be sure to check out my other shape3d Tutorials.
Shape3D#1: Different image on each sides of a cube.(EASY)--about the "Box" funtion.
Shape3D#2: Easiest way to make realistic eyes.(EASY)--about the "Sphere" funtion.
Shape3D#3: Use Cylinder to do Tree/Wood/Stems.(EASY)--about the "Cylinder" funtion.
Shape3D#4: Easy realistic Flower.(EASY+1)
Edited by Ego Eram Reputo, 04 April 2013 - 04:00 AM.
Updated forum links to plugins.
