Usually when I'm about to finish a new piece I go ahead and begin the background on the next I have on deck. This way I have time to let it dry and it will be ready the next time I'm ready to paint. As you can see from the photograph I printed it in black and white. This helps my process since my mind needs to be able to choose different colors according to the lights and darks I see in the photo. Next I do a very quick sketch on the canvas with an oil pastel to act as a guide. I try not to get into detail; it helps save time.
Once I have it mapped out I can begin the process. Many portrait artists have different methods for beginning a face and they are all great. My advice is to choose a method that works for you or change it up to get some good experience if you don't have a preferred method nailed down. I begin by framing the face with the hair which in this picture is the focal point. This helps me find everything else, eyes, nose and mouth. As you can see I started choosing colors. Remember they asked for Earth tones so I tried to stick to colors that were warm and complimentary; green with red, yellow and purple. These felt natural and so I went with it. Don't be afraid to go for it. Its easier to correct it if it feels wrong than it is rack your brain on choosing colors. The spontaneity is what gives the painting in this style energy and life. I felt that I was half way through so I decided to come back and finish another time.
Note: Typically I can finish a piece in about four to five hours with preparation. This includes drying and finishing.