Beginning with a darker value base also encourages a better range of tonal values, as you don't have the expanse of white paper that makes even light lines seem strong. It is less daunting to make a dark mark when there is already some tone on the page.
Apply the charcoal lightly and evenly, gradually building up a smooth even layer of tone, working in a variety of directions to even out the texture as much as possible. You can use a rag to gently blend it, but don't apply too much pressure - you don't want to push the particles into the paper fibers, as this will make it hard to erase.
