I'm no expert in this field, I'm an amateur, but I can give you a blow-by-blow guide to how to create reasonably priced simple facial prosthesis for Halloween or other occasions.

I began honing my skills in a local haunted hay-ride(Trails to Terror at Highland Farms), they basically said "here's the stuff, you do it." So through trial and error I learned the way of making freaky prosthesis.

This instructable still under construction. Sorry I wish I had a more visual approach, and I will, for now it's going to be mostly words. Any suggestions are fully welcomed just shoot me a message. Thank you very much and I hope what's here is helpful.

Step 1: Procure the Necessary Materials

So, what you'll need is simple

Liquid Latex: This stuff isn't exactly cheap but you won't need a huge supply. I used about 2 liquid ounces of the stuff over the course of 5 separate haunts.
Note:It's a lot cheaper to buy in bulk online, it's cheaper online even if you DON'T buy in bulk.

I Paid: $19.99 American for a 4.5 oz bottle, I overpaid from a costume shop.

Toilette Paper/Paper Towel: This is found in just about every household, I even nabbed some from a local grocery because I was broke and didn't feel like stealing from family. This will provide a matrix for the Liquid Latex to adhere to and build on.

I Paid: Free

Foam Sponge(s): This is for the more squeamish in applying make up/liquid latex, as I have found plenty of success of using just my fingers(however is a larger mess)

I didn't use, but you can buy some for: 8 sponges for $4.50

Optional Items

Cotton Swabs/Artificial Webbing: These items are easier to manipulate than just paper product.

Mirror: I would greatly suggest using one of these, however useful this is in creating symmetry in your prosthesis it isn't needed especially if you're not making it on your face.

Blow Dryer/Large Fan: This helps harden the latex quicker, air dry does just fine though.

Clay/Flat Non-Stick Surface: This is if you want to make the prosthesis off of the face. I prefer not using these because it's a snugger fit if it's made on the face. Clay comes in useful when it comes to sculpting more complicated effects.

Vaseline: Not for a moisturizer or lubricant, this time you choose to use it as a way to keep all original hair on your body.

Make-up: to hide you can use whatever color you see fit,

Step 2: Planning

Now you can do this quick and dirty, but the product generally is of poorer quality in comparison to those that are planned out at least a little bit.

Look at the subject to be modified(yourself, a friend, a foe?).

Go over the facial structure and decide what kind of prosthesis you/they want.
Scar
Burn
Gash
Slash
the possibilities are only limited to your creativity.

I would strongly suggest using a hat/bobby pins/bandana/wig cap to keep all hair out of your face while applying any make up/liquid latexunless you are bald than you have no real problem.

I look to enhance prevalent features such as cheeks/around the eyes/forehead

I try and avoid areas with high movement or other difficulties, the corners of the mouth/ areas around joints/ areas with lots of hair you don't want to remove.

I will go through prettying myself up for the haunt to show you how I do it.

Step 3: Create a Base

Now that you've planned what you're going to do uncap your Liquid Latex and apply a layer to the area you plan to use. Rub the liquid in small circles making sure that you create uniformity in the layer of Liquid Latex.

Make sure that you cover slightly more skin than you plan to modify: doing this does two things, it is a more natural blend from skin to prosthesis(most store-bought prosthesis have a "Clean Cut" which draws attention and is a pain to cover up with make up) and it makes sure that you can remove the prosthesis from the work surface without damaging the detail.

Let dry, this is where the blow-dryer can come in handy.

It is an odd sensation to blow-dry your face and not your hair.

Now apply another thin coat of Liquid Latex, just enough to aid in the adhesion of the toilet paper.

Apply toilet paper by covering the area and dabbing the paper down getting rid of any unnecessary bumps/wrinkles and let dry. It is a lot easier if you use a little more toilet paper than you need in order to cover the whole area.

Rip any toilet paper that is not stuck to your face. Rip away from the middle of the area.
It doesn't have to be pretty and if you have any little edges sticking up you can use more liquid latex to stick them to the face.

This toilet paper will be the mantel for the whole prosthesis

TIP
If you are applying directly to face, cover any hair that will have liquid latex on it with vaseline. It is a bitch to pick at dried-up liquid latex from your eyebrows/hair/eyelashes for a week.

I've learned that less is more: Use only what you really need to get the job done correctly otherwise your prosthesis doesn't look as natural and sticks out from the shape of your face too much

Step 4: Start Forming Your Prosthesis

I use 2 layers of toilet paper(base layer/latex layer/toilet paper) and then start forming. I plan to reuse the prosthesis, if you don't I'm sure building immediately on the base layer will be fine.

If you plan on just making a wrinkled effect: Apply toilet paper and scrunch up face. Having a blow-dryer handy makes this a lot quicker and easier(as well as reduces your chances of a face cramp). Keep face scrunched up while the liquid latex and paper is still moist.
This is a surprisingly very effective way of aging your face, you can get some real detail with the latex.

Scar Tissue/Gash: You can use two methods.
Method 1: For straight scars roll a piece of toilet paper in your fingers to create what looks like a rolled cigarette. Make two if you want to create a gash(One for each side of gash

Apply liquid latex to the base coat and start to stick on the rolled paper piece starting at the top end and working your way down.

Gash: Stick the second piece joining the two pieces at the top and then work your way down, leaving a space between the two pieces.

Method 2" Get your cotton/webbing out and remove a piece of cotton.

Pull at both ends of the cotton twisting your fingers in opposite directions.

If you pull both ends apart it's OK just roll both pieces between you fingers and start over.

Apply starting at top and tapping the cotton down
TIP: make sure you don't have any liquid latex residue on your fingers or the fibers will stick to your fingers and prosthesis ending in a huge mess and possibly some swearing.

Now you're going to have the top of the piece still dry and fluffy... this is the part where you're a little more liberal with the liquid latex.

Get a good drop of liquid latex on your finger/sponge and apply to the top making sure you dampen one spot at a time. Make sure it's enough to dampen the cotton immediately or the cotton will stick to your finger/sponge.

If you have extra from one spot work it down towards a still dry spot, but work out and down on the sides making sure not to pull away the cotton fibers.

I almost always go with two layers of cotton so that it is more prominent as I work in the dark.

If you are satisfied with the amount of scar material move on to shape it.
first flatten it against the skin and push one side up so as to fade the scar into the skin.
dab the cotton lightly with my fingers to make an uneven surface

Now if you are creating a wrinkled face scrunch your face and blow dry, or sit and wait for it to dry.

You can add another piece to make it look like a gash.

AT THE END OF BOTH METHODS: Apply another layer of liquid latex and toilet paper, when that layer dries add a last layer of liquid latex and let dry. Make sure to scrunch face every time you add a layer if you're making wrinkles with the scars/gashes.

Sore/Blister: Apply a layer of liquid latex and toilet paper to the base.

Before the liquid latex dries use your thumb and forefinger to pick at the surface lightly and form the open sores.

You can do layers if you want, get creative with it.

You can use all three of these effects together, just make sure that any sores are on the top or else they won't be noticeable and scrunch your face with every layer if you want wrinkles

TIP: you can make bullet holes and gashes with the same technique as Method 2 Scar just form the piece of cotton into a circle or apply strands of cotton as you go.

Step 5: Finish the Piece Up.

To finish all you need to do is add a layer of toilet paper and then let dry

Use a layer of liquid latex so that the piece looks just like skin and not dried toilet paper.

You're done!

Step 6: Taking Off the Prosthesis

You have two options

Ripping off: This option is easy
Without any regard for the prosthesis just rip off any piece making sure to remove any large pieces of latex(smaller pieces will come off with a good scrubbing).

Saving the prosthesis for later: Now ever prosthesis that I've made has had about a grand total cost of $2 to make. So I like to keep them if I can

Start at one corner of the prosthesis and pick at a corner until you have lifted slightly. Peel the prosthesis off as you would a band-aid, slowly but surely. Make sure you try and get as much extra as you can because you can always trim the prosthesis for the next time you use it.

Step 7: Reapplication

Use step 3 to form the mantel of the prosthetic.

Apply a coat of liquid latex to your piece, making sure that you cover every crook and nanny of the piece, air bubbles in the reapplication can be uncomfortable and won't be as secure as the first application.

Apply at the top and work your way down tapping out any bubbles with forefinger.

While waiting for latex to dry try and limit movement of the area so as not to create any air bubbles.

If you're ever in the Southern Rhode Island area in the month of October come on by and join us an evening, it's always a good show!

Trails to Terror at Highland Farms