My usual approach is to stump down the dark pattern with my fingers until I achieve a flat, even semi-luminous tone. But for this drawing, as I mentioned previously, I want a rougher effect. Using a #8 bright bristle brush I literally painted down the dark forms in exactly the same way that I would scrumble in the darks with oil paint.
For those of you who are new to painting this is an excellent practice for handling the brush without the added concerns of pigment and medium (i.e., turpentine and linseed oil). Be sure to hold your brush far back at the handle rarely should you paint gripping the ferrule of your brush.