"Inspiration" Graphite, White Charcoal and Watercolor on paper
While working on my drawing "Inspiration" I utilized a process that helped for modeling on toned paper.
This sphere study that I made before starting my drawing shows how I went about using my materials while modeling.
I used the tone of the paper to represent the similar value of a plane on the sphere turned away from the light source. All areas on the form that turned down from that plane became darker in value, while all areas that turned up (towards the light source) from that plane became lighter in value. Essentially for my portrait drawing, any part of the portrait that was darker than the tone of the paper became darker with graphite and any part of the portrait that was lighter than the tone of the paper became lighter with white charcoal.
Scott Waddell wrote a very helpful article about modeling on toned paper, which can be purchased at ArtistsNetwork.com and Anthony Ryder has some very helpful advice for drawing on toned paper in his book, The Artist's Complete Guide to Figure Drawing.
Also, I used photographic references for this drawing and found it useful to take many images of the portrait from various angles to help with a three-dimensional understanding of the form while working from the photographs.