You can check out some online videos.  I use them to help with problem areas.  I've learned not to look at it as an eye but each aspect as something different - eye ring, pupil, reflection in pupil, etc. Like the eye itself is a drawing with different aspects.  If that makes sense!

I usually put a large circle than a smaller circle for the pupil. Inside the pupil I put a small (Tiny) circle at the top of the pupil, which will be left blank, because it is sunlight or glare caught in the eye. The first large circle would be yellow for a grackle, brown for a sparrow. Obviously, the pupil would be black.