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
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.