Custom contact lenses
1. Face mask
With the polygonal lasso tool draw a selection around the eyes.
On the main menu go to Select > Inverse.
Copy and paste this on a new layer and name "facemask".
2. Cutting & pasting eye
Go back to the eye layer, with the Elliptical Marquee Tool draw a perfect circle around the eye of the subject. Copy and paste the eye on a new layer and name this layer lens.
3. Color and Gradient style
First choose the color you want your eyes to be, I choose a gradient of light green to dark green. Then turn your blend mode to "Color" Then choose a pattern for your eyes, this will become the design which will take over the white area of your eyes. I choosed the pattern named "strings" which should come default with your Photoshop.
Here is finished tutorial:
Practice with different patterns and colors.