Note: I'm writing this in England, where we write 'moulds'. If you are in the USA, you are probably more used to the spelling 'molds'. Please read as whichever you prefer!
Rigid moulds can be made of plaster, ceramic, metal or baked polymer clay itself, and since we have the clay to hand when we're working with it, that is my usual preferred medium. Although in the examples below I use a single, plain colour of clay for my moulds, this is so you can see the work easily - usually I mix up my scraps of colours from other projects and use that for the mould. This way I never waste any clay, even colours that have been mixed wrongly, got a bit grubby, or even little pieces left from a mixed colour that I'm unlikely to use again soon.
Bear in mind that with a rigid mould, any one part of the mould cannot incorporate undercuts - that's where part of the model goes out and then in again, like crooked fingers, or a downwards-pointing nose or beak - because you would not be able to pull the object out of the mould without breaking off that part. To get around this, you can make multi-part moulds, with the join going along the edge of such parts.
Flexible moulds can be made at home, too, and are suitable for complex shapes, even those with minor undercuts. I'll tell you more about that later.
Technique - Mould for Flat-Back Items:
The first thing to do, of course, is to find a suitable item to make your mould out of. You could make something yourself, or find a flat-backed object to work from, such as a metal charm or a button. If you use an existing object for your mould, consider whether it has a current copyright - generally, if you are just making things for the pleasure of crafting, no one will mind you making moulds from just about anything, but if you plan to sell the results don't use buttons, jewellery or other 'designed' objects.
In this example, I'm using a vintage metal button (right). Knead your clay until it is soft, then make a ball or egg shape and flatten it with your hand. The resulting 'blob' of clay must be thick enough to take the depth of your item, and large enough around to take the whole shape. If the shape is irregular, you can cut some of the spare clay off before you bake the mould, so it isn't wasted.
Brush or puff a release agent such as talc or cornstarch over the top surface of the clay, then press your object in firmly, making sure you apply even pressure over the whole thing (left). I prefer cornstarch, as it is water-soluble so it can be washed off later, if necessary, and it's not so bad if you breathe a little in by mistake. Don't press it in further than the flat back, as that would give your mould an extra layer of thickness at the back that you probably don't want. Alternatively, if your object is totally flat on the back (my button has a shank I didn't want to cut off) you can put the object on a flat surface and press the clay down onto it.
If your object is clay, metal or something else that won't be spoiled by the setting-temperature of the clay, put the whole thing into the oven without removing the original. This avoids the possibility of stretching or distorting the clay before the mould is set. If the original is plastic or similar, ease it out gently supporting the clay as much as possible as you do so. If it sticks, or otherwise distorts the mould, squish the clay back up, use more release agent and try again (kneading a little talc or cornstarch into your clay won't matter much).
When the clay is baked and cool, remove the original object (if it's still there) and you have your mould ready to use (right). See the doll project, below, for tips on using single-part rigid moulds.
Technique - Multi-part Mould:
I'm using a little plastic owl pencil-topper for this example, because as you can see, the wings stick out (and I like owls). Although it has a flat base, if I tried to make a one-part mould from this owl, the underside of the wings would get caught when I tried to take it out. Since I can work out one line along which there are no undercuts, I can make a two-part mould - if I needed more than one line, I'd make three or more parts, but the principle is the same. In this case the line goes around the owl, along the wing edges.
Make the first part of the mould just as for the flat-backed item described above, but be careful to push the clay up to the line where there are no undercuts - draw it on, if you need to do so. Before you bake or remove the original prior to baking, use any tool to make some fairly deep dots or gouges into the face of the clay, not touching the object. I've used the handle of a paintbrush. Note that because my object has a flat base, I've put the base at the edge of the clay. This will make it easier to use the mould, but can't be done with all objects (see flower beads, later).
When the first part is baked, test it out (there's no point in making the second part if there's something wrong with the first) and then replace the original object into the partial mould. Take another bit of clay, and dust the surface with release agent. Press this onto the object in the partial mould, pushing it against the face of the partial mould, too, so that it gets into the gouges you've made, causing bumps on it's surface around the original. Don't let it wrap around the sides of the first part of the mould. If your original can take the heat, bake that as it is. If not, gently remove the new clay, remove the original, place the new clay back on the baked part (it will fit into the shapes you pressed into the first part) and bake the lot. The release agent will prevent the two pieces of clay from sticking together. (Click picture for a larger view)
When cool, use your two-part mould by dusting it well with release agent again, putting in enough clay to fill the space, and closing the two halves so that they fit together. If you had a flat base up to the edge of your mould, this is easy because most of the excess will come out through the hole, and you can cut it off before removing your moulding and trimming any clay that squashed out at the seam. That part, on the seams, is called flash (see below), by the way. If not, you will need to experiment with the clay amount, until you find the right quantity to put into the mould.
Clay owl with "flash" visible
Finished owl with "flash" removed (Click for a larger view)
I took two mouldings from my button mould, above, and cut them so that I just had the flowers. After baking, I added clay between them to make a 3D shape with a flower on each side, then made a two-part mould out of that. I could then mould the beads shown, to match the brooch made from the one-part mould. (Click Picture for a Larger View)