Edit: I made a photo tutorial, so now its not just words!
First, will show you shots of my prototype, then of the actual eye on the head. Then, I will tell you how its done.
The prototype was done on a 25mm wiggle eye, and I didn't take much care when creating it, as you can see.
On the mask itself, I used a 30mm wiggle eye and took care when creating it. ts placed on cheesecloth in a mask where I see out of the corners of the eye.
So here's how I did it:
Step 1: Cut into the backing with a penknife. Be careful not to scratch the clear plastic itself!
Step 2: Stick your penknife in and begin removing the backing as carefully as possible. You may have to trim the backing further after removing.
Step 3: Remove the backing, and the piece of plastic inside.
Step 4: Using your penknife or a pair of scissors, remove a bit of the clear plastic from a side. This IS necessary, and you will have problems later if you don't do this step.
Repeat steps 1-4 for a second wiggle eye.
Step 5: Score the plastic of ONE dome with a penknife so paint will stick.
Step 6: Paint. Use paint markers, acrylics, sharpies... A note, if you use a permanent ink marker, you must first paint the dome with white paint. Otherwise, it will be transparent.
Step 7: Add in your pupil. Try to get this as centered as possible; otherwise, the effect may not work.
Step 8: Carefully center the two pieces, one clear and unmarred, the other painted, and glue around the edge. If you get it too thick, trim the glue with a sharp pair of scissors when it dries. Be sure NOT to cover the vent ports.
Step 9: Test your eye.
Finally, attach to your mask with hot glue and cover with eyelids or whatnot.
NOTE: During this last stage, you will notice that the hot glue has caused the eye to fog up. That's what the ventilation ports are for. Give it some time, and the fogging will go away.
You can even paint the remaining edges of the backing, or the entire lower dome, to make sure that there is no ugly laying around.
This tutorial used two 40mm wiggle eyes.
And voila! You have some pretty decent, awesomely cheap follow-me eyes. So far I have noticed that, since the eyes are not a perfectly round surface, if you get a camera flash too close to the eye it will white it out with light. However, I don't foresee people often getting that close to your eye.