Plaster of Paris
Mix the plaster according to package directions. Take a plastic spoon and skim off any excess bubbles on the surface then pour or spoon the plaster into the mold. Tap the mold on the counter to allow any unwanted air to escape. Allow the plaster to harden. This can take several days depending on how thick the mold is.
Make sure the plaster is completely set, and then pop the piece out by gently holding the mold with your thumbs on top and fingers on the bottom of the mold and flexing it out and up.
You will need to work quickly before the plaster starts to set.
If you are going to use a mold that is deep or has undercuts you will need to pour the plaster in several stages while still working quickly. Pour a little plaster into the mold and move it around making sure you have it in all the detail before pouring in more.
Paint plaster using any brand acrylic paint. It is a good idea to give it a base coat (white or off white is fine) and allow it to dry, before giving it a final coat. The finish will be much smoother and it will be easier to paint with color.
A product called Art Plaster is much denser than regular plaster of paris and will accept better detail. An even better product is called Permastone; it is highly refined plaster which will produce a smoother, more firm piece. Either of these can be found in any craft store.