Drawings are made up of lines and values. To create the illusion of form you need values or tones of grey, although the illusion of form is not the aim in the drawing below. The quick sketch below was produced using a couple of grey marker pens and a Pentel brush pen.

The photo reference is pretty much in silhoette and could be sketched as a notan, just black and white. I eventually decided to sketch it using four values one of which is the white of the paper. The key point in a value sketch is to reduce the number of values down to as few as possible - just enough to describle a general value structure.
It is a good exercise to do some value sketches using only 3 or 4 tones - black, white and a couple of mid tones.