Depends on what you mean I guess.
If you want torn up nasty burnt flesh, do a latex build up over the endoskull.
If you want Ahnold with his flesh half ripped off, exposing the endoskull... You'd have to make a copy of the skull to make a core, sculpt on top of that with clay, mold it, run silicone, paint, and transfer that to the finished endoskull... And I am way over simplifying that process.. it's definitely not as easy as just wrapping it in some magic flesh material though sadly!