Here is a step by step with pictures to kind of give you an idea of how I work with the pencil.

When I start working with the eye, I start first with the outline.  You can freehand the eye, or if you would like something to help you get the proportions correct faster, you can use a grid, or a projector.

Once the outline is ready, I usually just start working with different things around the eye, such as the little lines under the eye.  I used a 2B pencil here, working lightly with the edge of my pencil for the shading.



I then start working with the edge of the eyeball.   I look at the reference picture and ask myself, how dark is that? How dark is this part? How dark is that part?  Each question forces me to recognize where the shading gets darker or lighter:



I keep darkening the outside of the eye, until it is just about right, then I work on adding the pupil. 



I keep looking at the reference photo, which is enlarged on my computer screen, so that I can see the full detail and try my best to copy it exactly as I see it:



I leave a white spot where the reflection is in the eye.


Now, at this point, I get my tortillion out, and begin to smudge the eye until it is blended well, and looks smooth. I do not smudge the white spot.



Here is how the eye looks at this point next to the other eye...Kind of strange!




Next, I begin drawing eyelashes on the eye.  I barely ever take my eyes away from the reference photo as I try my best to copy them as I see them.  I try to be free with the pencil, letting it "wisp" to make the eyelashes, in a way that feels natural.



Now, I try to add some detail into the reflection in the eye. I pay close attention to the reference photo again and try to copy it.



Next, I get a kneaded eraser, and lightly try to bring out the highlights in the eye, making sure to brush it directionally in the way that the highlights go in the reference photo.



Next, I get the tortillion, and try to smudge the lines under the eye, so that they blend well and look natural.



Next, I work on shading the white of the eye.   I have noticed many people who do not want to do this part, because it just doesn't feel right to them. It seems like the eye should be "white" so they don't want to bother with the shading. But if they do not shade the white of the eye, they lose a significant amount of realism in the picture.  The white part of our eyes are shapes just like the rest our body, and they have shadows as well!

Paying close attention to the shadows in the picture, I shade, and ask myself "How dark is that part?", until I have it just about right.

 

Now, I am beginning to add the bottom eyelashes.



Here is how the eye looks in comparison to the other eye now:



Now, I just keep refining the details over and over till I am satisfied with the way it appears:



And then I add the eyebrows:



And there you go!  That's basically how I draw the eyes when I do my portraits.