I've been thinking a bit lately about orchestrating one of Debussy's piano preludes, I find that they are so colourful and just seem to be crying out fr an orchestrated version - particularly the Cathedral one and Voiles (both from Book I). The thing is, I have no experience whatsoever in this kind of thing. I have composed stuff for the piano and piano plus instrument before, but never something so large scale - would it be difficult to learn how to? Can anyone reccommend me a starting point or something? A friend told me to get my hands on a copy of 'Orchestration' by Walter Piston, but aside from that I'm a bit clueless really....

The Piston book is excellent. Everything you need to know is in that book!

Also, I would get scores to some of Debussy's orchestral scores (particularly La Mer) to see how he does it.

Colin Matthews recently orchestrated some of the preludes and these have been recorded by Rattle and the BPO.