Here is a simple technique in which i learnt through the shaped painting material project. Under painting is single layer of paint which is applied to the board, this serves as a base for further layers of paint. this is useful to define colour and give a slight tint to the painting.
Here is an example of under painting in my work. I have used a dark blue colour as i wanted to have a slight tint of blue in the skin tones.
In the last image i have began to paint in the highlights using a dry brushing technique which help me see the shape of the face.