1. Prosthetic grade alginate
2. Some sort of container to put your hand in (like Tupperware)
3. Plaster of Paris, Hydrostone, or HydroCal (preferred)
3a. A metal whisk or some other mixing tool you wouldn't mind throwing away.
4. Shellac or mold soap and warm water
Now I could explain this all day long but I figured I would show the video that helped me a ton in this, and many other, processes. They very clearly explain everything and have a good quality video so you see what is going on. The video is done by Brick in the yard Mold Supply and their webstore http://brickintheyard.com/ is where I get all of my mold making supplies.
Word of warning, make sure your fingers are spread apart....sounds silly but if you are wanting to sculpt in between your fingers keep that in mind.
It is very easy to do and does not take very much time at all. For Kerrigan, it is one of the few things you can actually do by yourself. After you have your alginate mold in your container, do not try and take it out. Leave it in the container for now, it is basically the consistancy of jello so the last thing we want to do is get rid of your plastic support shell. Something I tend to do is leave about 1/4 inch ring around the top of my container empty. You will see why in a bit. Now you take your plaster of choice, I would recommend Hydrocal or Hydrostone just because it is a lot denser and stronger than Plaster of Paris. My first hand cast that I made I did it out of Plaster of Paris and in the demolding process 3 fingers snapped off and I had to try and stick them back together. I got them back on but it was no longer pretty nor sturdy. On my left hand cast I used Hydrostone instead and no fingers broke off and I was very happy about the results. Follow the mixing directions on the packaging and take your metal whisk, or other mixing tool and mix very thoroughly!
This is VERY IMPORTANT! If you do not mix thoroughly enough it could result in a weak casting that can break and you have to start all over!
Tilt your hand mold slightly to the side and begin slowly pouring in your plaster. You want to tilt it to the side and pour slowly because we do not want to trap any air bubbles resulting in an unfinished casting (like missing the top of one of your fingers!). Now earlier I said that I normally leave about 1/4 inch off the top of my container empty on the inside. I do this to be an overflow for my plaster, as well as making a plaster stand for my hand to sit on all as one piece. So after you have carefully filled up the mold with plaster now all you have to do is wait. Different plasters take different amount of times to set up. Never, never, never demold it early! You will regret it! The plaster will heat up so don't be alarmed but after the certain number of hours it is ready to demold!
Seal it with either some Shellac (spray outside) or mold soap and warm water. Wait for it to dry and then you are ready to sculpt something!
Hydrostone lifecast + sculpt
Plaster of Paris lifecast + sculpt
Sculpt casted in silicone and quickly applied (better makeup/pics coming soon XD)