Paint the entire inside of the eye black.
Then edge the eye from the front then the back with a medium flesh pink. Use the same pink to accent the inside eye lid if it's showing. The front should be wider pink than the back corner and up higher around the circle of the iris in the front as horses most times are glancing back over their shoulders than directly in front of them. A horse's best expression is one of nearly being startled or having a spirited, alert, slightly wild look in the eye. The flesh should be painted in a 1 to 5 o'clock position on the front and a 7 to 9 o'clock position on the back corner.

After you've edged the flesh, take black and retouch up the roundness of the eye as you may not have the flesh perfect the first time on. Make sure the black is left wet for the third step. If it dries reapply some more black thinly.

Using a medium to dark fudge or walnut brown, start in the center of the eye on top of the wet black and blend lightly the brown from center to edge leaving some black towards the edge of the iris. Since the black is wet it will blend smoothly. What you want is a slightly marblizing effect with the wet paints. Go all around the eye dippling the brush tip lightly tapping the paint on delicately to dot the brown on the iris.

[sorry for the blurred photo]
What you do now while the brown is still wet (a good tip is to have all your eye colors out and open, ready to dip into.) take some of the same flesh used for the celera and delicately tap in a marblizing fashion into the bottom half of the wet brown. Don't do the entire eye to the top of the iris as it is supposed leave the dark shadow of the upper lid and lashes that cast over the eye. You're just highlighting the brown eye on the bottom half with the flesh and allowing it to marble itself into it by being wet. Almost in a crescent shape or half moon. You only need to go as far as the center, so technically you're just doing the bottom half of the eye. A good example of this would be the above photo of Moon Over Xavier and Asadd.

Take some black on the brush and gently tap and dot in the center of the eye the elongated pupil. Start at the left and work right. You should do this while the iris is still wet but if it is dry already, then use thinned down black so it wont be as prominant. Dot the pupil in in an up and down dotted pattern leaving a tattered edge to it. Horse eye pupils alot of times are not sharp in shape. They are marblized. The very center is solid black but the edge of the pupil is ragged and in a leaky marbled pattern. Like it's running down into the brown all around it. So tap in the long left to right shape but dot it up and down and zig zag it. It's rather sparatic. Refer to the above photo of Doc's Miss Buttercup as an example.

Now take a tiny heaping brush tip and dot your center pupil over top of the dotted ragged pupil edge. You can let your eye dry before this step. It doesn't have to be wet because there is no blending in this step. You put a wet accent of black across the pupil still leaving the tatter edge showing. You just do the center. It's ok if your paint isn't smooth. You really don't want it to be. It can be thick in places on the eye because you're adding gloss and it will hide it all. In fact, the thicknesses of the paint layers aid in showing depth and all sorts of shadow accents in the iris after it's glossed making it look VERY real.

Now using viking blue and with the very tip of the brush, edge the outside edge of the iris where it meets the celera. If you mess up, it's ok, because you have a deep black edge around the pupil you can retouch if need. Add just an indication of the eye leather membrain around the iris.
(Depending on what breed of horse you're doing the celera can be flesh with thinned down white glazed over it in a haze, or if you're doing Arabians, I was told this little tip by the breeders of Wunderbar Arabians that the desired look of the celera of an Arabian should not be white. The eye of an Arabian should be as dark as possible. It is the goal of Arab breeders to get away from bright eye whites. So the celera of a true Arabian should never be glistening bright white. Do it in flesh.)

Other celeras can be done in white then gone over in very thinned down black to give it a duller haze and not so piercing. Or sometimes a very thinned down flesh glaze over a white celera can give an indication there is real blood circulating inside the eye.

Add the highlights. Look at the horses eye in the light to see where the white highlight haze is. That's where you should put it. I normally do two highlight marks as it makes the eye look more alive. It really isn't even nessesary when the eye is glossed as the eye will sparkle naturally but in darker situations and in times when there may be limited light to the model the painted highlight completes the expression. I normally do a tiny dot in the top left or right area in either 11 or 1 o'clock positions and just below it and most times diagnal from it, a smaller dot. If you mess it up you can usually touch some black to it to reduce the size as it should be within the pupil area, so it's quite forgiving.

I don't recommend using clear gloss nail polish as it has a tendency to yellow and also the chemicals in it can cause the acrylic paint to break up and start smearing and running if you brush stroke over it too much. It's just not reliable. It's also hard to apply without ruining a small brush or making an utter mess with it. When I started out I started with clear nail polish but I've found something far better.
Delta Ceramcoat Polyurethane Gloss Varnish. Sold at Walmart for just a couple of dollars. I use it for everything. Even coating a base with it to affix sand and rock. It dries quickly and stays clear.

When applying this, what I do first is take a small amount on the brush and paint the whole inside of the eye with it in a thin layer, just to give it a even coat. Also to make an area for it to stay on and not have a tendency to run out of the eye. Then take a tiny drip on the end of your brush and while holding the model level gently place the drip onto the eye. Make sure the drip is the right size. You should be using a tiny detail brush for all of this. If you need to practice on some paper making little drips to get a feel of how much comes off and how far it spreads do so. It is thick enough that it wont run hardly at all. The only way it will run is if you pick up too much. You can get a feel for it with a few practice dabs. Once you know how much to use just literally set the tiny drop onto the eyeball. Hold the model for a few minutes level so it wont run or if it's not too thick set the model up and let dry. Do the same to both eyes and once dry, repeat if desired. Just set the drip onto the eyeball leaving it domed. The gloss will dry from the outside edge in and act as a small dome of glass over the eye magnifying all the marbling you did in the iris and pupil making the eye look alive, wet, and real. The gloss is what makes the eye come alive.

Now, You're done.