I started out trying to do an accurate value sketch, and it made the process easier. Using Picasa or any other simple photo software can convert a picture to black and white easily. This strips away the distraction of colour so that you can see just the values.
Although... I ended up thinking that the sketch captured the values more accurately. It was harder to be as faithful to the values using the paints.
21 cm x 27 cm, Arches 140.