This is another frequent question I get. As per my usual, I have no easy and quick formulas about this one either. (I know) Here are a few notes and things I have experienced about those windows to the soul. Nothing set in stone, mind you.
First off, understand the structure of the eye. This applies to 'learn the skills: draw draw draw'. Once you have practiced drawing the eye in all its manifestations, you will be able to more readily discern what pieces to let go of while painting. (this also applies to all of representation.) You don't need to paint every lash, every catch light, every wrinkle visible. Make decisions.
No two eyes are ever the same. This includes the ones on the same head. We've all heard that our faces are not symmetrical. Believe it or not, I finally realized I was struggling for YEARS to make those eyeballs THE SAME. What holey hell I put myself through doing that. It was almost as if my hand was trying (oh so hard) to tell me to stop it! I've come to understand that this concept is actually key. It's what gives life to that person; the imperfection, the way each eye can speak differently. When we look at a person face to face, we look at one eye at a time. Why not make that single eye a unique reading?
The edges are softer than you think, the whites are darker than you think. I typically begin with a warmish color in the socket, and then gradually build up to darker value using the neutrals. For me, that would be raw umber, or any mid range gray of some sort will do.
The whites of the eyes do not become a cool color right away. I use it sparingly in fact. It gives a hardness to them when those blues come on too strong. As does too much descriptive detail. Mistakes, inconsistencies, correctly placed (I know) are the ticket to real-ness. This only comes with experience. Once you have painted countless eyes *to the tee* correctly, it will become intuitive, and you will feel it. I do know I cannot seem to create them in one pass, it has to be more like 22. Glaze carefully for depth and more softness.
Notice the length of things. Vertically, and horizontally. Look for simple shapes first. How thick is the eyebrow? How far down is the top brow bone from that? The lid, the lash line, the iris, the fold under the eye? Then horizontally, left to right..the shape of the whites, the shape of the iris, the other side of the white, the tear duct? Soft edges. I cannot stress that enough.
The darks are lighter than you think. Save the darkest darks until the end. You may not need them. Full value range isn't always necessary, especially with eyes. Be selective where they go. That's the focal point. Sometimes a slight dash or dot will do. You'll only know if you try it.
Compare the values, shape to shape. Eye to eye.
Step away from the piece. Look at it in a mirror. Do the eyes follow you? If not, you have described too much. Wipe it out. Try again.
Ratty brushes work best for me. The ones with hairs missing. The teen tiny ones do nothing for me. I only use them for a quick indicator of the pupil, then dab with my finger to smudge it. I have often yanked hairs out of them to make them work better.
The catch lights are different in each eye, but in the same general area in both. They are never pure white, and are very soft. Think LIQUID. It has to move like the water it is over a sphere. We tend to paint them too hard edged, and this may look alright in a photo, but in paint, it looks wrong. Decide where your catch lights are, and don't put in too many, it will flatten the space if you do. Keep it simple.
The glazing is often endless. Be patient. This is the key area. Spend some time obsessing over them, without fixating. (I know) Make a few eye marks, then move around your painting to other areas, or you run the risk of contrivance.
Don't think: "I have to add emotion". You already ARE emotion. Nothing to add if you just relax. You can't decide what viewers will feel. It's none of your business.
In your spare time, just stare at eyes. Even your own. Only then will you truly understand.