I put down the middle toneThen I go from the general middle tone to a slightly darker tone, without going full contrast. That way I control the tonal relationships with out getting too dramatic. Ya know the old saying "Subtlety in tone creates the illusion of depth, while high contrast flattens."
I go back and lay in a lighter tone than the general middle
From there I go back and forth progressing toward the to extremes of the tonal range
Cycle back and forth until the localized tones relate to the overall and.... viola. Ya know. Of coarse along the way I'm constantly adjusting the drawing with the rendering
You can pick up all of this stuff by studying some of Sargent's unfinished paintings. He's one that seemed to relish in the process. He never tried to hide his stroke.
Now that I look at it... This caricature kinda sucks. Shoulda done a demo on a good one. Oh well. Another time then.