Here is an underdrawing on Claessens universal primed rough canvas, with five coats of gesso. I spend a lot of time preparing the surfaces, tweaking the color and sanding-back. This one is painted with Winsor & Newton gouache in ochre and brown. One has to be careful with the underpainting: it needs to be dark enough to cause the white highlights to "pop," but not so dark that the final painting is murky. It must be warm enough to provide a glow in the shadows of the final painting, but not so yellow that the skin appears jaundiced. Underpainting can make, or ruin, a painting.
And below is the finished oil sketch.