Finding the Darkest Darks
Begin by looking for the darkest parts of the shadows, these parts will usually be found where forms meet and make a crease or crevice. Such areas receive no direct light and very little reflected light which makes them nearly black. They are called occlusion shadows.
Reinforcing the Bedbug Line
Another part of the drawing which will tend to be very dark is the bedbug line (sometimes referred to as the terminator) – the dividing line between the lights and the shadows. This part is dark because (much like the occlusion shadow) it doesn’t receive any direct light and it usually curves away from the reflected light as well.
Lightening Reflected Lights
Next, you should lighten the areas within the shadows that are receiving the most reflected light. This will depend on the direction of the light source and the forms surrounding the figure. However, the light is usually above the figure so most reflected light comes from the floor and wall (if the figure is next to one). Light can also be reflected between forms within the figure – so for instance, an arm might reflect light onto the torso if it’s hanging down next to the body.
You should use a kneadable eraser lightly to remove tone; either as a flat pad, to remove large areas of tone or made into a point by rolling it with your fingers, to remove precise points of tone.
Evening the Shading as You Work
I recommend using the process of adding and subtracting tone to the shadows as an opportunity to even out your shading or introduce hatching that appeals to you aesthetically. Ideally this will be the last time you need to work on the shadows, so go for a finish you’re happy with.
Form and Planes Within the Shadows
You should also think about the forms within the shadows, as depending on the direction of reflected light, different planes will be lighter or darker. This is the same as forms lit by the light source directly, except the transitions between planes in the shadows are usually softer because the reflected light will bounce in lots of different direction, creating a more scattered, and subsequently softer illumination.