Here's a simple little 'ible for making repeatable wounds, cuts and bite marks etc.
The one piece moulds are made from casting plaster, then you can cast in latex, silicone or gelatine.

Materials and preparation:

You need a clean, non-porous, flat surface to work on. I find an old melamine covered kitchen cupboard door to be an ideal board for this. It also means you can move it aside when things need to dry.
Plasticine, or another oil based non-drying modelling clay
Some WD-40 or lightweight baby oil
Plaster of Paris, Herculite, Crystacal R or another similar gypsum based casting plaster
Basic modelling tools to sculpt your wounds
Liquid latex, or silicone rubber or a gelatine mix (there's a recipe for this later in the 'ible)
Some skin safe attachment glue such as theatrical latex, spirit gum, Pros-Aide, Mastix or similar

To finish, some stage blood, face or body paint and make up. You may also want to have some talcum powder or baby powder handy.

Step 1: The Basic Sculpt

Here I am going to make two very simple sculpts just to show you the techniques.
If you only wanted a single non-reusable wound then you could just do these directly on the skin using derma wax, skin safe silicone putty or gelatine. However this 'ible is about making re-usable and multiple copies of the wounds. You can make a few up and give them to your friends!

Lets start with a vampire bite, these are very easy to make. Take two small balls of the modelling clay, make sure that they are fairly similar in size. Plonk them down onto your work-board and then push down the edges all round to create two low mountains of clay. Don't make them too big, somewhere under and inch or 25mm should do it. Now take a plastic sculpting tool, or a blunt pencil and make two indentations in your clay mountains where the fangs have punctured the skin!
Once you have your two puncture marks use a little WD-40 or baby oil on your finger tip and smooth the edges of the two bites so that they will blend into the skin well.

The longer you take working on the sculpting part the better the end result will be. Wipe any excess baby oil from around the sculpt taking care not to disturb you beautiful work!

Next we will also make a cut wound. Start with a 'sausage' of modelling clay and again plonk it onto your work-board. Smooth the sides down until you have a long, low ridge of clay. Then take a sculpting tool, or the side of an old spoon and make a cut down the centre remembering to leave a little uncut clay at each end. Work the sides of the cut until you have a nice smooth finish, continue to play around with it until you are satisfied with the result. Remember not to make the ridge edges too high or it won't look convincing.

When you are satisfied, use the baby oil or WD-40 to smooth and dissolve the edges so that they will blend into the skin once fitted.

Step 2: Making the Fence

Now we have our bite and cut sculpts complete we need to make a retaining wall into which we can pour our plaster. You could make this in a number of ways, using more modelling clay to form the fence, or from strips of plastic stuck together, but by far the tidiest and easiest method is to use standard Lego bricks stuck down with double sided tape.

The fence needs to have at least a 1 inch (25mm) gap all the way around the sculpts and needs to be deep enough that we have about an inch of plaster thickness for our mould. We could probably get away with something thinner but this way we know it will be strong enough, and plaster is relatively cheap.

Step 3: Mix and Pour the Plaster

Many different types of plaster are available for casting. They are mostly based around Plaster of Paris and some have quite confusing mixing formulations. I have been using these gypsum based plasters for many years now and have come to the conclusion that the following mix will work correctly for these small moulds perfectly well.

2 parts plaster powder (by volume)
1 part water (by volume)

This could not be simpler. Take three standard plastic drinking cups and fill one with water to the brim. Fill the other two with powder to the brim. Pour the water into a plastic mixing jug, then add the powder little by little stirring all the time. You should end up with a mixture the consistency of double cream. Try and make sure that all the powder has been evenly distributed through the water and that there are no lumps. Now tap the jug a few times to bring any bubbles to the surface.

Because of the melamine, the oil based clay and the Lego we shouldn't really need a mould release agent for this job. However, if you have some mould release spray there's no harm in giving it a light coating. I tend to use silicone release spray for everything, mostly because that's all I usually have handy.

Start to pour the paster mix into the mould at one edge, this should allow the plaster to flow out evenly into the mould. Don't rush it or pour too rapidly, you want the mixture to flow into all the details without trapping any air bubbles (that would be bad). Pour the entire contents out into the mould until we have a depth of about an inch. Slightly more is no problem but if the plaster is too thin it may crack when we de-mould.

Now I would leave it alone for 3 or 4 hours to give it a chance to set off well.

Step 4: De-moulding the Mould

Since we used a wall of Lego, de-moulding these small plaster ones is easy. Simply dismantle the Lego walls, you will invariably find that small fillets of the plaster have seeped into tiny gaps between the bricks. If the plaster is dry it is very easy to wipe these bits off the shiny plastic before you store them for another job. Try and make sure the bricks are properly cleaned before you store them so that they will fit back together well next time.

Once the fence is removed you should have a nice, tidy plaster block. Gently twist it to break the seal and then lift the plaster away. Some of the clay may be stuck to the bench and some may be stuck to the plaster. No matter simply use an old credit card to lift any clay from the board then give it a wipe over ready for next use.

Gently try to remove any clay from the plaster mould. At this stage the plaster still contains a lot of moisture and is quite soft, even if you used a hard casting plaster it is all too easy to damage the surface. Any stubborn fragments of clay can be removed with a soft brush.
Only as a last resort should you use any kind of plastic, wood or metal tool to try and get the clay off. However, it must all be removed before we can use the mould.

Once cleaned set the plaster mould aside for at least 24 hours to give it a chance to set properly.

Step 5: The First 'pull'

So we now have our clean plaster mould. Time to see what results we can get from it.

Here I am going to use a simple slip or skin latex for the first pull. If you want to try gelatin instead (if your model is allergic to latex for example), mix the following ingredients in a plastic jug and microwave in 20 second bursts stirring in between bursts until the mixture is liquid and all the gelatin has dissolved. Then pour the mixture into the mould and leave it in the fridge for four hours. You can then just peel the 'wounds' out and glue them on with spirit gum or similar.

Gelatin mix (all by volume):

1 part honey
1 part water
2 parts gelatin
2 parts glycerine

For the latex pull I am simply going to paint 6 thin layers into the mould recesses allowing each layer to dry in between. You can use fewer, thicker layers but that takes a lot longer to dry. It's OK if you go over the edges by a little, that will give you more surface area to stick.
Once cured, dust with talc then simply peel the wounds out of the mould ready to start on another set of pulls. remember to dust the backs with a little talc as well, this will prevent them from sticking to themselves.

Step 6: Fitting the Wounds

If you used latex to make your wounds then I would recommend using theatrical latex as your adhesive. Remember some people are allergic to latex so check first! Get a small amount of latex on a piece of old cut up bath sponge and dab it onto the skin on an area slightly bigger than your wound. You only need a very thin layer.

Then use your sponge with a little latex on it and put a very fine layer onto the reverse edges of the wound. Wait for both lots of latex to cure (they will turn transparent). Than gently place your wound onto the skin and it will bond instantly. Smooth down the edges so that they blend out into the skin. If required, further blending can be achieved using some latex on a Q-Tip to run a bead of latex around the edges. Once it as all cured use a little talc on a blusher brush to dust over the edges ready for paint or make up effects.

If you used gelatin or skin safe silicone, then use one of the theatrical glues like Pros-Aide or Spirit Gum - follow the manufacturers instructions but generally you put a thin layer on the skin and a thin layer on the prosthetic then wait for the glue to reach the very tacky stage before attaching the wound.

Gelatin edges can be dissolved away with a little witch hazel.

Again a light dusting of talc will help with the final set.

Once in place use make up, fake blood, or face / body paints to make your wounds look gruesome and realistic.

Here is a simple bullet wound prosthetic made using the instructions above. Standard make up, fake blood and face paint has been used to complete the look.
Image courtesy of Cindy Van Mieghem.