When it comes to creating fake skin for special makeup and costume effects, there's no material more standard than liquid latex, and for good reason. This stretchy, moldable material doesn't need much help or alteration to resemble natural human skin and takes on color and special effects alterations well. However, the process of making liquid latex into convincing fake skin still requires knowing the ins and outs of handling this material correctly.
Choose a base on which to apply your latex. Latex will dry flexible but take the shape of the surface it's applied to, so it should either be painted directly on the body of the person who will wear it, or painted on a piece of tight-fitting clothing while worn, which will then become part of the costume (recommended). This can include a pair of leggings, a hockey mask, a small T-shirt or a pair of gloves.
Coat any exposed skin on your subject with petroleum jelly. Pay special attention to any areas with hair. The jelly should be applied thickly enough to leave a sheen on the skin. This step is particularly important if you are applying latex directly to real skin, but is often necessary even beneath clothing (as the latex can sometimes seep).
Apply a base coat of latex to the cloth or skin. Spread the liquid with a paintbrush or sponge (whatever you use will be destroyed by the latex, so don't use a good brush) in an even layer. Let dry, or speed up the process with a hair dryer.
Create or choose paint or makeup colors for your skin. Though latex can have paint and makeup applied to it after drying, mixing the color directly into the liquid will create the most even color. It will also allow you to match color if you're going to be using makeup on bare skin near the latex pieces.
Mix your color into your latex. The latex is opaque when wet, which will make your colors appeal paler and creamier than they will be when the latex dries. Knowing how much color to use can often be a matter of guesswork, but if you use about as much color as you would to thinly paint the surface of the finished product, you'll have a good start (you can always correct the color with an outer layer later).
Apply your layer of colored latex directly over the base coat. Pop any air bubbles in the latex. (Unless you want to leave them in for a gruesome, skin-melting effect. If so, you can create more air bubbles in your latex by putting it in a container and shaking it.)
Create special effects in your wet latex skin. Soak pieces of facial tissue or cotton in your colored latex to create a clay-like mush that can be used to sculpt warts, scars, ridges, or any other shape you want into the fake skin. Be sure to coat generously and don't leave any parts of foreign materials uncovered (though if you do miss a spot, you can cover it later as long as you keep some of your colored latex covered and wet).
Allow latex to dry, or use a hairdryer. If using a hairdryer, use a low setting or a diffuser to avoid a blast of air misshaping your latex creation.
Lightly coat the surface of your dried latex with a coating of talcum powder or cornstarch. Latex, even when fully dry, remains naturally sticky and will stick to itself without this aid.
Dried latex can easily be trimmed with scissors, so err on the side of going too far when spreading it on.
Use liquid latex in a large, well-ventilated area.