In order to make an exact duplicate of someone's hand, the easiest (or at least cheapest) thing to use is called Dental Alginate. This is a seaweed-based product used by dentists to take impressions of teeth and gums in order to make dentures or retainers or the like. The material itself is sold in a powdered form. To use it, you mix it with water and it turns into a gooey mess with about the same viscosity as ketchup. After a few minutes, it hardens into a rubbery solid. It's safe to use in direct skin contact, and you can control how fast it cures by changing the temperature of the water that you mix it with. Warmer = faster. In order to make a copy of a hand, you mix up a batch of alginate in a bucket big enough to give you at least half an inch of clearance all the way around the appendage in question. Then you stick your hand into the bucket, wiggle it around to work the bubbles away from your skin, assume whatever pose you want the duplicate to have, and wait until the alginate solidifies.
Here's a shot of Jenny showing us what her hand looks like:
After mixing up a batch of alginate, I had her insert said hand. Here she is waiting for set-up time:
Once the alginate has solidified, you need to gently remove the hand by wiggling and twisting and pulling until you break the vacuum and you can slide out.
Here's Jenny wiggling (always a fun thing to watch):
Once she'd slid her hand out, this was the hole that was left behind:
The little bumps you see on the left side of the hole are the pores in her skin. The alginate picks up every fine detail. So much so, that the resultant castings will actually have fingerprints that are identical to hers.
The one drawback to alginate is that, once it cures, it will shrink as the moisture dries out of it. So it's important to pour the casting as soon as possible. In this case I used black casting resin:
During the pour, you have to tip the mold down toward the fingertip side in order to let any trapped bubbles out:
Once the resin sets up, it's a simple matter of breaking apart the alginate and getting the cast resin piece out:
The result: a plastic hand identical to the original:
If you're making this same project at home, you need to repeat these steps as necessary for the number of hands your model has.