I believe it is called trial and error. I watched my sister and of course read tons of magazines. But I have to say that most of my learning came from make up books, I must have read Kevyn Aucoin's (aww I miss him!) beauty books about a 100000 times, Making Faces was my bible. I would try to recreate the looks (and fail) in the book over and over. He had a gift that's for sure! I still reread Making Faces and Face Forward over and over expecting something new. They don't make books like that anymore!
As I am still learning (hey you can always teach a weirdo dog new tricks!) I go on youtube and watch the beauty gurus, which now are too many to count! I seriously do not know how they do it, I could never put myself on camera and do my make up without looking like a complete moron from being so nervous. I'd end up looking like this: