There are many different materials today that can be used to make prosthetics, but most of them can be hard to find and complicated to use, but gelatin is available anywhere, and with a little practice, is very easy to work with.
Unflavored, unsweetened gelatin powder.
Hand mixer (with only one beater)
skin colored acrylic paint
flesh tone makeup
Step 1: Sculpting
To begin, use some plasticine clay to sculpt your prosthetic on a paper plate, in my case I'm making a puzzle piece shaped wound, from the 'SAW' films, but this can be used for just about anything (burn scars, bullet wounds, gashes whatever). Make sure to smooth out the edges of the sculpt as best as you can, the smoother you blend it into the plate, the easier the piece will be to apply later. cut the bottom out from a plastic container, and place around your sculpt, seal edges with more clay.
Step 2: Casting
Spray some release agent (i.e. cooking spray, PAM will do) on your clay prosthetic and the walls of the container, just enough to very lightly coat everything. Mix up your plaster, pour in just enough to cover the sculpt, then tilt and shake your plate so that the plaster gets into all the nooks and crannies, then pour in enough plaster to fill the container about half way.
Allow your plaster to dry thoroughly, when the surface is cool to the touch, you'll know it's done. Carefully remove the plaster from the clay, you might destroy your clay original in the process, but were done with that now any way. If any bits of clay are stuck in the mold, run a little bit of warm water in it to soften it up, and use a metal pick (paper clip would suffice) to get the bits of clay out.
Step 3: Prepare Gelatin
(note: I was so focused on this part that I neglected to take pictures, sorry)
The amount of gelatin you will need to use is relative to how many prosthetics you will be making, If your only making one, a single packet should be plenty. The amount of water used should be half as much as your gelatin powder. Pour your packet into a plastic cup, add a tiny bit of your paint and your hot water. Start your mixer on low and work up to high speed. The gelatin will expand to about two times it's original size, and lighten in color.
Once you have finished mixing, spray some more release agent into your plaster mold, and pour in some of the hot foamed gelatin, just enough to cover the surface. stretch some saran wrap over top to thin out the edges.
Allow your piece to cool for a few minutes, then very very carefully, remove your finished gelatin piece from the plaster.
Step 4: Applying to Actor
Start by having your actor clean the skin where the prosthetic will be applied. The cool thing about gelatin is that it can be partially melted with warm water. Wet down your actors skin with some warm water and stick the appliance on. By dabbing your fingers in more warm water, you can smooth down the edges of the appliance and blend them into the skin.
Step 5: Finish Piece
Using some flesh toned makeup, match the tone of the appliance to that of your actors skin.
Take a small brush, and paint some of your fake blood into the wound, add some blood dripping down as well.
Your done! Pretty good eh?
Step 6: Removal
The appliance and blood should remove easily under some warm water and soap.