I always begin with a rough pencil sketch, which I then play around with until I've the form I'm after. Then it's erasing and shading until I'm happy with the image. I then use charcoal to bring out the tonal drawing, and when I'm satisfied with what I have I go in with pastel chalks and pencils.
This is the part I enjoy most, the painting seems to come to life as each layer of colour is added.