I'm going to assume you already have the base you want to use: cavalry or monster. My advice is to just grab a picture or two of the GW troll that fits that base (Cave troll on the cavalry base) and just size it the way you want from there.
On a more specific note, I've sculpted a troll myself, with some technical advice from ebob. Start the beastie with a wire-frame skeleton to make sure he's in the pose you want and build onto that. I used miliput for everything but the outer layer, but I'm sure there are other solid putties out there as well.

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