I like to begin with a full-size sketch on paper. This serves two purposes: First, it gives me a chance to experiment and adjust the composition before I start painting. It's much easier to change a pencil sketch than it is to change a painting. Second, the sketch serves as a kind of "warm-up". It helps me to get the feel of the subject.
When the sketch is finished I know exactly what I want the painting to look like before I start work on it.
I trace my sketch onto a panel using graphite transfer paper. For portraits I like to do a traditional under-painting using raw umber and white paint mixed with Liquin. It's essentially a "black and white" image. It doesn't bear much likeness to the sketch yet, because I've only blocked in the basic shapes. At this stage I'm only trying to establish a basic pattern of darks and lights, but it's important to cover the entire panel so these value relationships become clear.
The panel and the Liquin are important to my technique. The panel's smooth surface allows me to create fine details that would be obscured by the weave of a canvas. Liquin is an alkyd painting medium made by Winsor and Newton. It lets me create translucent glazes that dry overnight.
After the first coat dries, I begin a second coat, still working with raw umber and white. This time I try to establish a likeness. This is where it gets difficult. The first coat of paint is now covering the outlines of the sketch, so now I work entirely by eye.
Sometimes it takes many sessions to get the likeness right. In this case, I struggled with the eyes and the mouth. Here they looked too sad and stern, which was not my intention.
I continue fine-tuning the features until they capture the subject. Here is where the time spent on a good sketch pays off. Paint doesn't handle anything like a pencil; but the features remain the same, and working on the sketch trains the eye to recognize the nuances of the subject. It's amazing how much difference moving a line a millimeter makes in a portrait. The eyes now appear to slight twinkle, and the mouth seems about to break into a smile.
Finally I add translucent layers of color on top of the under-painting. For skin-tones, I start with reddish layers and then apply more yellow color over that.
When you view final painting, you're actually looking through the upper layers. The light shines right through the colored glazes, and bounces off the lighter areas of the under-painting. This is how the Old Masters achieved glowing colors in their work.