Yes, you must decide when to act
This technique is as simple as it is possible for anything to be. You paint an oil wash over unfired water-based paint (undercoat and trace lines). Blend smooth.
Optional: add dark oil shadows and blend again.
And then while the oil is wet, stipple – because the oil is wet, the effect is gentle (see eye on your left side).
Now wait maybe half-an-hour (or whatever) for the oil to dry a little (it is absorbed by the unfired water-based paint beneath): stipple some more, but not everywhere: the whole point is, contrast. The effect starts with pin-pricks (which because the oil is dry, they never heal), and the more you stipple, the more paint you now remove (see side of face on your right; also the top of the nose).
Yes, timing is
everything a lot.
Important: you’ll find the design for this head and 8 others in this guide here.