Theatrical makeup is both a skill and an artistic outlet. The skill lies in creating the lines and shades you need to apply, and assembling the makeup kit you're using in the first place.
Lines and shades you can practice with a pencil and a sketchpad. You'll need a sketchpad anyway, because you have to create the makeup for every character you're working on: who are they, how old are they, what kind of hardships have they had before the production? You want distinct faces, and that requires creativity.
Assembling the makeup kit is what you do at the local drugstore, not from the "makeup kits" sold on Amazon, because you can't spend huge amounts of money on every character and make this work.
A good starting place: will take you through the basics of putting a kit together.
You still have to add spirit gum and some samples of crepe hair for mustaches and eyebrows as well as a spirit gum remover. Finally, you'll need to have some fake skin and fake blood for wounds and scars. Mehron liquid latex is probably a necessity unless you're really into creating combinations of loose powder and vaseline stiff enough to last under the lights (hint, it's hard).
Finally, you need some heads with stands (not real ones, ones you can show the actors their makeup with). Most actors will look harder at your face with a stand than at your sketches, so take the trouble to do it right. Faces with stands cost between $9-$20. I've generally gotten along fine with just the front of the face to show what needed to be done.