Before I go too far, I should say that you have already studied the figures face, it's expression and where you want the eyes looking. Are they straight ahead, looking sideways, or down, etc. So you should paint the iris's appropriately.
Once the iris colour has dried, I then take a slightly thinned amount of Burnt Sienna and work it from the corner of the eyes, touching the bottom of the iris and moving into the other corner. I do this top and bottom of each eye. Does it matter if you obliterate all of the white??? No. The whites of the eyes are usually the downfall of modelers attempting to do figures for the first time.
When you have an iris isolated in a field of white, is when you end up with the unrealistic 'stare' of a bunny in the headlights.
Try an experiment. Hold a 35th scale figure in your hand and then ask a friend to walk away from you until they are the same size as the figure you are holding. How much detail can you see? Not very much at all. This is the trick of modeling where we include detail that normally you would not see, but done well, it gives the subject depth and dimension.
Incidentaly, I apologise for the quality of the photos, but I only have my phone.