The only materials you need besides paint: balls and a cutting tool. I used a snapable box cutter, fully extended. I should also add that you should buy at least twice as many balls as you need, because you WILL screw up a few times, and one ball = one eye.
There should be a seam in the plastic, and below that you can see the edge on the paper image inside. Pick the best looking side that you want to use (probably the one that doesn't say "made in china"), and plan to cut along the surface of the paper on the OPPOSITE side.
This is the side I want to use. There will be resistance, but try to cut as smoothly and straight as possible.
The other side. You can see what I mean about cutting along the surface of the paper.
The left is the bad side, because it's hard/impossible to cut one of these without leaving marks inside.
Horrible picture, but this is the cut side of the good half. Carefully cut around the image with the intention of removing it and not puncturing through the other side. Peel it out.
Now you have a nice mark-free surface to paint on.
Paint the pupil first, let it dry then paint the iris, keeping in mind that whatever you paint will be magnified. I used acrylics. Be very carefully when the paint is dry, because it WILL want to peel off.
<one pupil is bigger, I know x_x
Well, I hope that was helpful to all those other instant gratification type folks out there.