One of the best ways to practice and develop your use of your tonal skills in watercolours is to work in one colour. At the end of the exercise you count up the amount of tones you have achieved, I usually achieve ten. You do not want to achieve say three as you are going too quickly from a light tone to a darker tone.
Select the colour you are going to work in and then you make as many washes as you can: 1% paint/99% water, 20% paint/80% water and keep going increasing the amount of paint until you reach water 1%/paint 99%, which is more of a mix of course than a wash.
Make a line drawing of the face and hat. To achieve a likeness you need to measure, measure and measure. One line out of place and you instantly lose a likeness.
Start by painting the hat. You have to make this look like soft material and to do so work wet on wet. Wet the paper and wet it again. Apply your weakest wash of colour, then two more washes of colour. Then take your mix, use a large brush and dab in the dark shapes.
For the skin tone: use your weakest wash and go over with the next weak wash you have made. Use a strong wash for the eyebrows, shadow around nose and mouth.
Paint in the eyes with a dark tone, and also the hair on the side of head.
The clothing: work from light to dark, using all your washes for this. This part of the painting took me some time as you need to work in glazes, where you let the paint dry between subsequent applications.
Paint the background with a light wash of colour, to take away the starkness of the colour of the paper.