i came across some amazing pumpkins done using a similar method and wanted to have a go myself. learnt a lot from this.
remove the skin of the pumpkin (try not to take off too much of the flesh when you do this)
you need to hollow out the pumpkin much thinner than for a usual carved pumpkin so the design will show through. i made the shell of mine about 1cm. i then left mine to dry out for 24 hours (depending on your pens you may not have to do this but i found a slightly drier surface helped)
draw the design with an oil based marker pen (sharpie do one...try ebay) if you dont have one (i didnt..
) use cd or permenent markers and keep scribbling on paper to keep the ink flowing. alternate pens frequently, even with frenzied scribbling its hard to keep the ink going. i went between about 6 pens.
a dry eraze white board marker works well but has a tendancy to spread so use sparingly on anything detailed. works wonders to cover large areas
(i only had one in green as you can see...but the colour wont show when lit)
i used a knitting needle to carve into the pumpkin to bring out a few highlights. because the skin is so thin you dont need to carve deeply with a knife or similar, a dried out biro works just as well
my second traditionaly carved pumpkin...lego wolverine!