Ok, let's get started. The first thing that I always do when starting a new portrait is the eyes. The eyes and lips carry the personality. Once you get those right, the rest of the portrait will follow in place.
I made a pretty big mistake when I made my initial sketch. I made the eyes a little bit too small. The problem is that I did not notice this until the portrait was almost complete, so I had to go back and fix it. If I had caught it early on, it would have been a lot easier to fix. So, check your initial sketch and make sure that you have all of the elements properly proportioned. One way to check for sure is to scan the sketch into your computer, size it to the size of your original, put one image as a layer over the other in Photoshop and then toggle the layer on and off. You will be able to see what is incorrect and you can then make changes.
The first thing that I want you to notice about the original image is the highlights in her eyes. Normally, both eyes will have almost the exact same highlights. However, in this case the source light was from an open door to Tanna's right. Therefore there is a sharp highlight in her right eye, but the left side of her face is in shadow, so the highlight in her left eye is much more subdued.
Also remember that you are not drawing objects, you are drawing lights and shadows. Don't worry about the fact that her eyes look wet. You are not drawing "wet." Once you draw the lights and shadows where they are supposed to be, the end result will be realistic looking wet eyes.
Start off by marking where the highlights are in her eyes. Then, using your 2B pencil darken the pupil, making sure to leave the highlights white. Once that is done we need to focus for a while on the iris. If you notice in the original image you do not really see her iris's very clearly. But one of the secrets to making eyes look deep is showing all of the little lines and details of the iris. So we are going to put them in there, even though we can't see them. Starting at the outside edge of the iris begin to make short quick pencil strokes inward to the pupil. You do not want to make your lines all the way to the pupil but rather about 1/2 to 3/4 of the way. Also, you do not want to make them uniform. There needs to be lots of variation in the length of the lines and the darkness of the lines.
The pencil stroke you use for the iris should be a short quick stroke that ends with lifting the pencil off of the paper. This will create a line that tapers off to a fine point. Once you get those lines complete, do the same thing from the pupil outward. This time you will be using a bit shorter of a line.
Now it is time to use the Tortillion. This is the key to making this entire technique work. With the fine tip of your Tortillion start rubbing the lines of the iris to slightly blend the graphite together. Take care to avoid the highlights. You will notice that this blending will seem to add "color" to the area and will soften the harshness of the pencil lines. They will begin to look much more like real irises as apposed to lines of graphite.
Once the irises and pupils are complete you can start moving to the whites of the eyes. It is possible that after using the tortillion there is enough graphite on it's tip that you can simply use it to actually draw the shadows in the corners of the eyes without even touching it with a pencil.
To complete the work in the eye, you should start working on the area of the face that surrounds the eye. Spend a lot of time looking carefully at all of the details in the area that you are working. A really good practice is to work in small areas at a time. Do not try to draw the whole face, just a small portion. Also, when laying down graphite, barely let your pencil touch the paper. What you ultimately want is to use the pencil only to put the graphite in the areas that you will be needing it. The real work comes in using the tortillion to spread that graphite out. If you make your pencil lines too heavy then you will not be able to blend them correctly and they will show through. So, start out very light. If you see that you do not have enough graphite on the paper to get the area dark enough, simply lay down another light layer.
If you have any questions, run into any problems, or need something explained in more detail. Please comment on this post and let me know. This is the first time I have tried to teach through written media. I normally teach one on one. So the more feedback I get the better I will be able to write each new post.