Obviously it's a work in progress, but Kymba has been doing a really thorough job covering just what she's doing with Sculpey (a brand of polymer clay). You can follow her own accounts on her Livejournal page (start over here), or check out the gallery below, in which we've compiled all of the pictures of the process so far. The jaw was made separately, and from what I can tell (I've never sculpted in my life), she used aluminum foil baked into the clay wrapped around a sturdier core. The whole thing looks great so far -- we can't wait to see what the finished product looks like.
Stay tuned -- you'll probably see it all done in a future World of Warcrafts post.
Gallery: Deathwing sculpture in progress