I like to say that this is not a technique to use on all the portraits you will paint, the best technique to get the most wonderfull portraits is the grisaille, but meanwhile you can practice with this few colors.
This technique is very simple, the most important about this is how you achieve a natural skin color with so few colors, here the colors you are going to use, titanium white, burnt sienna, ultramarine blue, that is it, we do not need anything else for the moment.
First we need to understand that this is not the way most profesionals works but if you are a beginner and you want results asap I think it is the best way to get it.
The color of the skin you want to get it would fully depend on the amount of white you add to the mix, if you want a very white skin it is obvious you need more white, little bit of sienna and almost nothing of ultramarine blue, but if you want a dark skin then you need less white more sienna and more blue.
If you mix just white and sienna you will get a very cruede skin color by adding blue you are going to gray the mix and the result would be a more natural color.
Before to start thinking about the skin color first we need to get a very structured face drawing, because if the structure it is not ok, not matter how good you get the volume or the color or whatever you can do to the portrait it wont look ok, be aware that the first stage is the structure after that you need to know a little bit about volume, because with volume you will get the tridimensionality of the head, eyes, nose an all the parts of the face.
Here is an image of a portrait I painted with this few colors, at the end you can apply some glazzes to get a more color variations.