There's no special prep work, just gut the thing and have at it. Make sure you carve it out GOOD though, if the skin is too thick, the light won't shine through (I even scraped more out after I was done with the design). And I happen to be one of those instant gratification people, so I just freehanded my design, but if you're one of those anal retentive types, you could draft something with a Sharpie first.
Personally, I enjoyed this technique more than traditional carving. I used to have issues with cutting just a smidge too far, then have to redesign on the fly, or pin pumpkin parts back into place. Not anymore. Try it out for yourself, I promise it's a cinch!