You'll need some white-out (a white-out pen with a thin tip is best, or a toothpick of applying white-out from a bottle), and a black pen with as thin a tip as you can find. I recommend a gel or drawing pen, but any good pen that writes smoothly should work.
Apply a tiny bit of white-out to the doll's eyes. This will be the catchlight. Try to apply as little as possible - you can never have too little when doing this. Wait a second to let it dry.
Use the pen to draw around the white-out to form the pupil. Try to keep as much of the eye color as you can while also not obliterating the white-out. Wait a few seconds to dry.
That's it! It's easy, fast, and works like a charm. I hope you found this useful, and if you have any questions or tutorial requests, I would love to hear them.