Experiment with makeup and supplies to see what works for you. It will take time to develop a feel for the makeups you'll eventually work with. Start with basic Halloween supplies, or buy a kit from a theatrical supply store. Use guide photos and re-create what you see. The best way to begin learning is to start working with makeup. If you can get a live subject, so much the better. If not, use a Styrofoam bust. Practice is the key.
Apply makeup with the same care an artist uses to paint. Practice blending colors and creating shading and depth. For theater stages in particular, these elements are essential for making actors seem larger than life and stand out.
Choose household items you can use to create effects. You can use oatmeal and latex adhesive to create the face of a rotting corpse. Red food coloring mixed with corn syrup is an effective recipe for blood. Learning to be creative on the spot can be one of the best skills a theater makeup artist can have.
Make changes to suit your needs. If you need a wig and can't find the appropriate hairstyle, find the closest match you can and cut it to fit. A theatrical makeup artist does more than just apply makeup. Like any artist, your goal is to create an illusion using whatever means necessary. Learning hairdressing skills, understanding how makeup colors blend and how to apply latex prosthetics are all part of the equation.
Make latex molds when possible. Learning casting and how to mix and pour the various types of latex used to make prosthetics and masks can save you time in the makeup department. Practice making original creations to hone your skills.
Practice removing makeup quickly and efficiently. By the end of a long shoot, actors and actresses can be tired and impatient. Learning to be as skilled at removing as you are at applying makeup is part of the skills a makeup artist should work on.