I had a question recently on the process of mold making.
Here is the most easy and painless way to make them: Go Talk To My Buddy Mitch Rogers!
He's the most sweet guy you'll ever meet and is super helpful. On top of it, he can explain to you exactly what materials to use for what job and so on. Mitch was actually nice enough to help me out with my Henry The Tortoise. I sculpted him and Mitch helped me with the mold, since I decided to do that on my own instead of hiring a professional mold maker. BITY (short of Brick In The Yard: his Mold Supply Company) has tons of options. With all the options it's very easy to get super confused, but I promise he'll talk you right through it all.
There are lots of different types of Molds (or Moulds) out there, but for our purposes, we'll stick with what I do.
For me, a mold is a some sort of a rubber or silicone, which is in a liquid form and usually contains two parts. When the two parts are mixed, the material can easily be applied with a brush onto a sculpted piece we want to cast in metal or plastic, or what ever else you desire.
It's much more complicated then this of course. There are the obvious issues with, lets say, a brushed on rubber mold: Take it off the turtle and the mold will flop. To fix this pesky problem you'll have to build a mother mold on top of your rubber mold. A mother mold is built from something that will turn hard, and therefore will be able to support the shape of the floppy rubber mold. I like to use a two part hard plastic or foam, or even plaster.
Another problem with a sculpture like Henry is that he's quite intricate... you can't just pull the mold off a piece like that. So what can be done about that? Well, the mold and mother mold will have to be engineered in parts which will be able to pull apart, but also stay together tightly for when you are ready to paint in the wax for the metal foundry or pur in some plastic for other purposes.
I'm sure your head hurts by now, so I suggest watching a few BITY videos... they'll greatly help you in your understanding of the process. I'm going to plug in a few below, but he's got many more you can check out on YouTube. If you have any questions, I'll try to help out.