Fingernails work, it's what I used when I did this a few times.  Your nails just get really gunky :\  Otherwise, a smooth-edged knife works pretty well.  Anything with a decent edge, not sharp.  Rounded edges don't work.<p>Overlaying with black is, yes, covering the whole page with black crayon.<p>So, the steps:<p><pre><code>  Fill ENTIRE page with colored crayon patches / squiggles.
  Color over ENTIRE page with black crayon.  Try to make it TOTALLY black.
  Scrape off with fingernail / knife / wooden clay tools / whatever.
  Hang on the fridge to admire for several generations.
</code></pre>
Ultimately, it's a cheaper way of doing those scratch-drawing things, and doesn't require sharp tools and a careful hand to not gouge the paper.