Here is a very low cost gelatine infected zombie bit prosthetic. It is less likely to cause any skin irritation if you are allergic to rubber latex but still do all the usual tests with the materials to check for allergies prior to application explained in the next step.
The whole process takes about an hour after a couple of attempts and you can re-use your negative mould multiple times as the gelatine washes out easily, as long as your Plaster of Paris mix is strong. Normal ratio is Plaster of paris 2 parts to 1 part water. Simple plastic cups are the best way to measure amounts required.
The most important part of any prosthetic is to ensure you have thin edges for effective blending to the skin. This example is a little rough and needed more blending but it was done just to show the process and less emphasis was put on realism but it's still fairly effective and hope it gives you the basic idea.
Please be aware: Gelatine is extremely hot when melted (like hot toffee) therefore please don't apply it directly to the skin or you may create a permanent special effect prosthetic.
Step 1: Allergy testing and preparing your skin
With all the products used in these instructables there is potential for allergic reactions. Certain common things may be unlikely, but paints, latex and other fx supplies have the potential to cause reactions. Large numbers of people are allergic to latex, and you do not want to find this out after you have just painted it all over your face.
The most common way to test products for allergies is to place a small amount (a few mm across) on the inside of your arm. This skin is very sensitive, and is also not visible normally, so not so much a concern if you do get a rash or staining.
You can do multiple 'dots' for your different products, and it is sensible to do this a day in advance so you can be sure there is no problem.
To ensure a smooth makeup application, it is sensible to exfoliate your skin first to remove the layer of dead skin on top, which will ensure that any prosthetics you apply will stay in place as long as you want them.
After exfoliating it is a good idea to moisturise. This is not only good for your skin, but should also help the smooth application of the makeup, and provide a slight barrier to prevent any staining that you might get from, e.g. fake blood.
When applying prosthetics to skin, or anywhere you will be putting liquid latex, you will most likely want to shave any hair (certainly on arms and legs) as it is extremely painful to pull these off when they are attached to hair. When applying Gelatine prosthetics they are easily washed off with warm soapy water.