I started off with a drawing, using a grid to help me place the features and to speed up the drawing process. A good drawing is an absolute must I reckon. I then put in some initial washes - a warm yellow on the left, a cool yellow on the right, and a blue wash for some areas of the face, hair and surrounds, all according to what I reckoned I could see in the reference photo.
I let the washes dry completely (this is where I often run into trouble with watercolour, being impatient and diving in too soon!). I've added some red/orangey and purple tones to the hair and ear shadow.
Again on dry paper, I've put in a reddish wash across the face, defining the areas of shadow and light. For the neck, which is pretty much entirely in shadow and darker than the face, I've gone with a purple wash. For the highlights, I either avoid painting the area altogether, or I use kitchen paper to blot away the colour where it's not needed, which actually produces a nice soft gradation for the edges, good for portraits.
More thin washes and blotting of highlights, and defining of shadows, under the eyes for example.
Yet another thin layer or two on the face, plus I added the background colour.
More layers to deepen the skin tones and blotting of highlights and pale areas on the face, and I've added texture to whatever it is she's hiding behind (something wooden, anyway). At this point it was looking a bit blotchy, but not to worry, it can be fixed with more washes...
...so more washes, drying in between, to smooth out the blotchiness a bit. This is the point I've left it at, though I think it still looks a bit blotchy, and I'm wondering whether I can go deeper with the skin tones without it getting overworked. But she does have a bit of a ragamuffin feel as is, so maybe I'll leave it...
Watercolour, 11" by 15". Reference photo by babasteve on Flickr, under Creative Commons.