The background was a nice mid-grey color, just because I find it easiest to work with than black or white. You should block out the eye and the area around it. Just something basic--nothing really fancy or special. Start placing in the white, starting with a low opacity and going over it. It can be painstaking, but it is definitely worth the process. Try to get somewhat the tones you'll use in the eye. Take the circular marquee, hold shift, and drag a circle for the iris and pupil. Such a simple step. Take the black, and with a low opacity, begin shading the white, and giving definition to the rest of it. continually shape and reshape the eye to get it as right as possible. Smudge time! Take this kind of... paint splat smudge brush, and start smudging things together. If you see any mistakes, go over it with the white and black brushes to fix it up somehow, adding highlights and shadows. Also, notice that the iris isn't totally sharp, slightly blurred around the edges. Think about the shape of the eye. Remember, the eye is a sphere, not flat. Also, adding a bright high-light at the bottom of this eye gives it the appearance of it being moist. For the iris, you basically use the same strategy, going over with white, then black in different opacities untill you get it right. I find this the hardest part for some reason. I corrected the eye shape and some of the shading at this time, too. The highlight will be the whitest of white. Lots of touch-ups here, and finishing it off with a scripply half nose thing. I know this tutorial was kind of rushed, including the pictures presented here, but the actual size (smaller one in the corner) doesn't look half bad.