when i worked at a prop shop, i had my arms, from the elbow down to the hands, cast for a frankenstein display. one of the owners wanted it for his house...
i sat down, with my hand hanging into a bucket, we then filled it with alginate and waited till it set...
to get my hand out, i had to wiggle it around a little bit, and slowly pulled it out...we then filled it with plaster,
we then had to clean up the plaster hands, youll probably get airbubbles due to the pose of the hands, then molded the plaster hand, and cast them in resin.
my arm got kind of sore, holding it still in the position i was in for so long. and that was the hardest part.