This is my basic process for drawing noses and eyes - i normally fiddle around with everything a lot more but exactly what i do is hard to explain - i spend hours tweaking with the colour, contrast, saturation and textures.
One thing i should mention is that i do almost every new highlight / shadow on new layer in case i don't like it and to make it easier to adjust. I gradually build them up until it's complete and then merge them all.
I don't like heavy shading - it's unrealistic unless there's harsh lighting.
Oh and everything here is just how i do it - i've had no training or anything so i'm probably wrong about lots of things. I learn basicaly by taking a reference of something (like eyes) in different lighting, angles and shapes and drawing it over and over again until i know the structure and shading of it. Then when ive done that i can be confident in doing my own from scratch.
Anyway, hope this helps
(I'm sorry for any spelling mistakes i'm in a rush right now.)