Yes, get a course
I don´t see anyone hiring a fx makeup artist without a course. You have to know a
lot of technique!
I have to say, i simply agree that you have to have a course in any makeup area.
Before i did mine, i had no pretensions in what i knew, but years of reading and practice, wouldn´t have put me where i am now if i didn´t do the course, and i just say this because i haven´t read anywhere the techniques i learned during the course.
And reading isn´t exactly the same as having someone near you pointing out the flaws and getting you to do things just right!
Of course like others mentioned, there is no formula, but that´s a bit like saying that you don´t have to study to be an actor. Yes, you may not need as many lessons, but good actors weren´t just born with that.
In makeup yes, you can practice, but lessons, a diploma and some good school pics for a first portfolio, will put you ahead of those who are still aspiring.
I really do think it´s hard to know the right ways to do things such as applying eyeshadow, when you are just practicing without knowing the basics.
...and yet, there are so many things to learn and so much practice to do.