|Here is a detail from the finished painting, enlarged because I used very small brushes.
In the next image, you can see the brushes I used. One small pointy one, and another splayed out and busted one. When doing shadows, I'd paint with Mr. Pointy then stipple and soften with the busted one. I'm not very fussy about materials or brushes. When I was young, I used to read all those manuals and study hard to learn the "secret" materials that would give me the edge. The only secret, I learned, is your eyes.
It's all too easy to fall back on what we "know" about Form rather than simply using the evidence presented before our eyes. "Shadows are dark; highlights are white; reflected light goes here; etc." These are all learned rather than observed truths. [Painters use old tricks such as flipping their image upside down, or examining their work in a mirror, to escape the natural tendency to paint what we think we see.]
I build the shadows very slowly using Ultramarine and Raw Umber acrylic paint, mixed with a little matte varnish (which dries quickly so I can keep working) as a medium.
The shadows are built up in layers. I never try to establish the darkest dark in the first pass. It's much more tentative then that. I build up darks in translucent glazes, always erring on the lighter side. I tend to work back and forth all over the image, as opposed to finishing each area completely as I go.
Use highlights very sparingly! As subtle as mine might look to you, when I look at the opening detail image of this post, the highlights jump out at me as being too strong and brushy. The image looks like it's been dusted with snow. The shadows are soft and muted, as they should be, but the highlights are harsh, overused and overly delineated. We want to avoid this at all costs. (It was too late for me).
It will be easier to see the differences between stages if you view these images in slideshow mode, and scroll between them.
I certainly didn't want to add any more highlights, as mine were already too bright. Instead, I used a very washy ultramarine/raw umber glaze and stippled it all around the top right corner, outside the main figure.
I also used a glazed version of my highlight color and subtly lightened the bottom left corner (again, outside the figure only).
To give you an idea of how light/dark my values are, here is a chart that shows you (below)...
Along the top you see 3 swatches of color. These represent averaged tones taken directly from the finished piece. Directly underneath them, you see 3 grey values (A, B, and C). These are the same colors from the top line desaturated so as to see value only. Below that, I plotted A, B, and C against the Munsell value chart. You can see that the entirety of my painting occupies roughly three value steps on the Munsell chart (from 6 to 9). No white, and nothing at all on the lower half of the chart.
Some more detail photos...