Preparing your skin for makeup
To ensure a smooth makeup application, it is sensible to exfoliate your skin first. What is this? Basically, removing 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.
There are plenty of commercial products out there, but the best thing is probably plain Baking soda. With a bit in the palm of your hand, add a small amount of water to make a paste, then rub on your face firmly to gently remove the top dead layers of skin.
After this 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.
If 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.