You can complete a course called "basic proofreading"
I wouldn't even bother with the courses, because without a long list of clients you have already worked for, it's a difficult market to break into.
That's a pretty vague and sweeping generalisation.
Let take a normal 300 page txt, how long do you think it would take, scanning every word, every punctuation, making sure the technical parts are spot on, checking that the grammar is fit and proper, and on and on.
Then you will be made to work within very small time scales, miss a deadline and kiss your ass goodbye. Another point is that larger firms will employ 2-3 different proofreaders, and from time to time the others will check on your work, another potential slip up.
Unlike teaching, which for me at least is the easiest thing in the world.
You can take my word for it if you like, I'm heavily in the business;)