I start out with a textured background and start laying down tone on a new layer. Using the Adonit Jot Pro stylus and an iPhone with force touch, I'm able to vary the amount of tone (force touch enables pressure sensitivity).
Once I find some shapes I like, I bounce between adding and subtracting values and refine my shapes. I introduce gray tones to separate the focal point from the background, keeping the background transparent and the focal point, in this case the face, opaque.
This technique is part of my workflow--one of my favorites. I'll often sketch out an idea, or study, before tackling it on a larger scale.