I have never made a plaster mold so I have learned a lot so far. Although I have read about making plaster molds, I still had a bit of a challenge.
First, the material used for build-up. At first I used polymer clay and it left a sticky residue. I was told I need to use water-based clay or oil-based clay. So I went and got Modeling Clay (Plastalina) which is oil-based. That worked out so much better.
Second, the mold halves were sticking to each other. I ended up buying Murphy Oil Soap and mixed it 50/50 with water. Brush it onto the plaster and let dry. Do many layers. What you are looking for is the point when the liquid soap mix can no longer soak into the plaster. You will notice the surface becoming shiny too. Soak up any excess soapy water before pouring the plaster.
Third, when you add the plaster powder to the water let it soak. Mixing immediately adds air bubbles into the mix. So just let the plaster powder soak in the water. Then after a minute or two start mixing.
Fourth, brush plaster onto object to reduce the chance of air bubbles getting trapped. Air bubbles on the outside of the mold is no big deal, but you don't want bubbles messing up your details.
Fifth, be patient. Let the mold set before messing with it. You can ruin your work by being impatient.
Here is what I used to make the plaster molds:
1. Plaster of Paris
2. Build up Materials (Modeling Clay (Plastalina))
3. Mold Release (Murphy Oil Soap)
4. Coddle or Wall to contain the plaster
5. Mixing Container (disposable plastic cup)
6. Mixing Stick (plastic butter knife)
7. Measuring cups (1/8 cup)
8. Paint Brush
I think that just about covers what I used. Some sites will tell you to have a bucket of water to clean your materials, but I use disposable items. You do NOT want to pour plaster in your drain pipes. You have to do what works for you though so if you need a clean water bucket that some tutorials tell you to have, do so.
EDIT: I do use a small container of water to rinse the paint brush in.
It is humid here in Florida especially with it being the rainy season right now, so I make sure to wait at least a day before casting the other part of a multiple part mold.