Sculpting a likeness is an amazing skill. Different sculptors have their own techniques and it is something that they make look easy but that they have worked at over time. Portraiture sculpting is all about capturing a likeness of someone and making it look realistic.
One technique to begin with is to start with some reference photos of the person that you are to sculpt. This includes a front view, side views, a back view and some 45 degree views of the face. Of course it is good to look at the person you are sculpting in the flesh but it is not always possible, especially if you are doing a celebrity likeness.
Another technique is to take initial measurements of the head and the facial features. Use a fixed point to do this, measuring from the BTE point (between the eyes) to the chin, the back of the head, the end of the nose, the inner ear and so on. This may make it easier to get an initial shape.
It is also important to think about how the face is made up. Think about the shape of the skull and how it has certain plains and aspects that are the same for everyone. Some sculptors may even start by building up the muscles on the face and sculpting over the top, although this would be a longer and a more specialised technique and is more used for sculpting the body. For my sculpt that I am doing of Sir Peter Cushing I had to think about the way that his cheek bones were very prominent, studying the shape of his Zygomatic arches as shown in this diagram below.
When sculpting patience is key. Sculpting with clay means it is easy to take away and put on material where needed so don't be precious over it, you shouldn't be afraid to make drastic changes. Also take breaks and walk away or cover up your piece so that when you come back to it you can look on it with fresh eyes and it will be easier to spot mistakes. Other advice includes using a mirror to compare both sides of the sculpt and asking the opinion of others who may be able to spot something that you haven't. One thing to beware of is to keep turning it and looking from all angles otherwise you find that your sculpt has a very flat face.