For high quality, long lasting moulds, most cold casting methods use an RTV rubber of some kind. These are excellent, and I do use them, but they are expensive and pretty slow to cure. Usually 4 to 24 hours. The polyurethane resin favoured by most small parts casters, such as Easy-Flo or one of the other 50% / 50% mixes are easy to use but need to be ordered in and again can be expensive.
I wanted detailed results VERY quickly so I chose to use Alginate as my mould making material instead. The resin is car repair resin, which I can pick up 5 minutes down the road.
Step 1: Materials
A pattern with very little undercut
Car repair resin
Some plastic cups
Stirring sticks (the ones from the coffee shop are best and free)
The problem with Alginate as a casting mould material is that it is not very strong, it cant handle big undercuts and it's considered to be a 'one shot' mould. Dentists use it to cast your teeth. The advantages of alginate are, that it takes very fine details, it very safe to use, and most of all it cures in 90 seconds flat. Also it is MUCH cheaper than RTV
You may have to think out of the box a little when making your patterns. I wanted to cast a set of small tank wheels. I had to fill some of the undercuts with plasticine, then grind them back with a Dremel after casting. I also built the main pattern so that there were no undercuts at all.