Clean the part of the face where you wish to apply the adhesive with rubbing alcohol and cotton swabs. You want a clean skin surface.
Apply a base color to your face matching that of the prosthetic you wish to use. This will depend on the effect you want to achieve--a sickly look may involve a gray or yellow base make-up, while a more normal look may be close to your own skin color.
Use a brush to place Spirit gum or similar adhesive down on the spot where you intend to place the prosthetic. Then lay the prosthetic piece over it until it sticks.
Apply base make-up to the prosthetic until it matches the color of the rest of your face. You want it to blend as seamlessly as possible so that the prosthetic appears indistinguishable from the surrounding flesh.
Add highlights as you wish using a stipple sponge or make-up brushes. Again, this will vary according to the look you wish to achieve. Natural skin tone may demand typical highlights, while fake blood can be applied using a stipple sponge for a wound or monster effect.
When you are ready to take the make-up off, apply make-up adhesive remover to your face, or use baby wipes to clear away the prosthetic. Wash your face carefully with soap and water, then use moisturizer or skin cream to keep it from drying out.