What follows is my first Instructable so please, be kind.

Any and all constructive criticism, comments and questions are appreciated.

Here is how I make one-off appliances, however you can replicate them indefinitely, which i will cover in a future Instructable.

This is what you get at the end. A big 'ol gash with muscle fibers and all that gory stuff that adds the detail you love.

This technique can be used to make nearly any type of injury, cut, gash, blister, boil.... what have you.
Your only limitation is your imagination, amount of materials, and patience.

Step 1: The Hardware

The Picture is a little cluttered, so heres a list of my necessary tools, you may like additional things, use what you like.

Tools:

Toolbox: it holds things in a relatively organized fashion
Heat Gun: dries latex, makeup, other things in a jiffy
Assorted Paintbrushes: I hope their use is self explanatory
Acrylic plastic: I got a piece of off-cut at TAP plastics for $1.50, it's 1/4" thick
Styrene: I use it as a palette, its cheap and you can clean it easily.
Scissors: Small surgical-style ones are great, the sharper the better.
Glass bowl: glass is good, can be sterilized and washed easily
(not pictured) aluminum foil, I line the glass bowl I pour my latex into with it, makes cleanup easier.
Sharpie
Stipple sponges: (coarse and medium) makes stuff look like skin.

Materials (for basic gash)

Liquid latex: the main ingredient, without this and its unique properties, none of this would be possible.
Nose/Scar Wax: Used to build up the appliance in certain areas
String: Replicates some connective tissue/fat/other stuff really well
Appliance adhesive: GLUE!!!!
Rubber mask grease paint: paint, it sticks to latex
Baby Powder: stops latex from sticking to it's self.
Makeup Fixer: stops makeup from smudging so much

Step 2: And So.... It Begins

Determine the basic shape you want for your wound.

I chose a gash for an arm, underneath the skin is a ton of musculature and veins and other stuff, detail is key in making it look good.

I begin by outlining my basic shape on the piece of acrylic using the sharpie.

To keep from contaminating your latex pour it into a bowl, I line my bowl with aluminum foil.

Using an el cheapo foam brush apply your first layer of latex, go 1/4"-1/2" outside your line. For blending purposes during application, you want the outer edge as thin as possible.

I made several lines of latex off to the left, these will become muscle fibers later.

Dry with heat gun on the lowest setting, moving around frequently so as to not burn the latex.

A WORD OF SAFETY AND CAUTION ABOUT HEAT GUNS

Heat Guns can be dangerous . They can shoot air hot enough to give you 2nd degree burns. The metal tip gets very hot and can cause a burn in a split second. Never use a heat gun near flammable vapors or anything explosive.

After drying first layer with heat gun apply 3-4 additional layers to the wound portion only, the lines need only one layer.

Step 3: If You Build It.....

Somehow I cant rotate the picture... oh well....

Apply prosthetic adhesive to your outline, this will adhere the Scar Wax to the latex below. Allow to air dry 5/10 minutes (dont use the heat gun for adhesive) have a drink of your choice.

Make snakes (and/or worms) of Scar wax and apply them to the now dried appliance adhesive, smooth the outside edge, it may help to wet your fingers with water (spit works well too)

Once you like the appearance of your gash, use a foam brush and apply some latex to seal the wax.

Put away the heat gun, if you use it to dry anything from here on out, you'll melt the wax.... which isn't good.

Step 4: Sinew and Muscles, Ewwwwww

Now we begin to turn what is kind of tame, a flesh colored oval, into the beginnings of a gory appliance worthy of shrieks of terror.

Move over to the lines of latex we painted way way way back on step 1.

If they aren't fully dry (like mine) it's okay

Begin by peeling the end up and twisting it in your fingers, you will cause the latex to stick to itself, creating a strand of latex, it should be imperfect and twisted.

Peel all the lines up. Remember to do the twist. These are now ready to become muscle fibers. YAY

Using latex' characteristic ability to stick to itself start to arrange the muscle fibers into rows.

Cut off a little string, rub it in your hands, pick it apart.... basically make it look as much NOT like string as possible

Put a little appliance adhesive on the muscle fibers and let it dry.

Place fibers from the string on the appliance, it should stick to the adhesive. Paint over with latex which will stick the fibers down more, let dry and apply more latex until it "looks right" which is totally subjective.

FINALLY when your appliance is "done" apply baby powder, use a big soft brush to get the powder into the crevices. This will keep the latex from sticking to itself.

Step 5: Paint Paint Paint

So this is the part that really makes the appliance look awesome, or not so awesome.

It is also the part that is the hardest to instruct on, either you get the concept or not, the goal is to make the wound appear as deep and appear as much like a hole in the body or limb as possible. While the appliance is in reality no more than 1/8" thick throughout the middle portion.

Also, I didn't really take too many photos of this process, I got too into it I suppose. Sorry.

Using rubber mask grease paint begin to paint the wound, get black and dark reds down deep into the crevices.

Fill in the main musculature with a medium red tone.

I like to highlight the sinewy stringy bits as well as some of the muscle fibers with yellow, it really adds depth to the appliance and helps sell the effect.

When it looks right spray the whole thing down with makeup fixer. This will help stop the makeup from smudging.

Step 6: Pull Pull Pull

The entire reason why I like using the scrap sheet acrylic to make my appliances on is that it makes removing them easy, the latex peels of without hassle at all.

Begin by finding a short stiff bristled paintbrush, this will help aid in removing the appliance (the one in the picture is my go to brush for this purpose, in fact its sole existence is now removing latex appliances. FACT)

pour out some baby powder and get the brush coated in it, begin to work the edge up, continue brushing the backside of the appliance with powder, after all this work you really don't want it to stick to itself.

When the appliance is fully removed, look at your work, appreciate it, and start the next one.

Lastly, you can use this for so many different effects, here are a few I have done in the last week-ish for friends and myself.

I will do Future Instructables on application, thanks for checking it out, vote for me, comment, question, scold me or whatever.

THANKS!!!

Step 7: Ouch... It Hurts.

As requested, here is a photo of the applied wound. All made up.

Using appliance adhesive put a thin layer on the appliance and let dry. Once dry stick to skin. Use a stipple sponge to apply latex and blend the appliance. Match your skin tone with rubber mask grease paint and blend the appliance. I used alcohol activated paint for the fine details and veins. Spray with makeup fixer and apply fake blood and blood gel to the wound to increase the gore factor.