When in doubt, always check FFURG too... There's usually a tutorial there listed on the site somewhere.
Here are the FFURG Casting Tutorials
Personally... Sculpey's not a good thing to work with as far as casting goes. You're further ahead, if you want to start into this aspect of customizing, to get a starter kit from a company like smooth-on and try casting with RTV Rubber and a simple 2-part resin. Not all resins are created equally, and some are quite a pain in the ass to work with... Smooth-On's starter kit is cheap, simple to work with (2-part silicone rubber and 2-part resin), and where I started years ago.