If at all possible, I highly recommend you paint your portraits using a live model as opposed to a photograph. There is simply no substitution for painting from life.
Painting a successful portrait is all about how you observe the subject. You want to study the subject as a whole. Study the bone structure and try to see shapes and planes. Do not try and paint every little detail exactly as you see it.
For beginners, it is probably best to start out with a lighting effect where light and shadow are in high contrast. This will make for a much easier painting.
Focus on one section at a time. Finish each section before moving on to the next.
Keep the darks of your portrait at a thin consistency while your lights should be painted on thickly.
Many beginners struggle with mixing flesh tones. I know I did when I first started painting. Remember that skin comes…