You need to spill the blood of the original caster's firstborn into the casting mold...

Easy enough to do, I guess. But who's??? I would bet Rob's, based on his first person descriptions and choices in this article that he wrote for the geek site. If the caster has no children, will spilling the original caster's blood work?

Rob: We won't need much. Maybe a pint or two. We can always get you a refill pint at The Wrestler's (or an equivalent location).