When you are drawing a portrait one of the most important thing is shading. There are two ways of drawing a charcoal portrait that basically determine your further shading process. You can cover the whole paper with charcoal powder, and then go from a darker shade to a lighter one using a kneaded eraser, or you can do it backwards, start with light shades and add more darker ones in the progress. If your goal is to draw a portrait as realistic as possible, then I recommend the last method.
You can't create an illusion of a real skin without proper shading techniques. But even knowing all the methods of shading and blending is not always helping, because artists usually hurry to get the final look of the portrait and forget about the patience. Two key words of the perfect shading are "moderation" and "patience". I want to tell you more about them and how they can help you to draw a realistic portrait.
The first key word "moderation", can help you to understand that too many shadows can be a disaster for your portrait. Don't try to find all hidden shadows or tones, you should focus on creating smooth transitions. On the picture below you can see the tools that can help you with that. They are a sponge stick (you can use a regular sponge) and blending stumps.
Also, don't forget about being patient. You can see from the pictures of the shading progression that the first two stages are not very beautiful or perfect. That's because you need to do shading steps by steps, adding darker shadows little by little. Artists usually rush this process, because they want to see the perfect results already on the first stage. But you should remember that it's much easier to add some more shades, than to erase them. It takes longer time, but this way, you won't get unnecessary charcoal spots, which are difficult to get rid of. Your portrait will be perfect and realistic.
I hope this tip is helpful for your shading process. If you want to know more detailed about the shading techniques please keep reading my blog, new tutorials are coming.