Your best bet is get a degree in English. The most common employment test for proof readers is to correct a document loaded with homophones and homographs.

Don't know what homophones and homographs are? You need a degree in English. While you're enrolled and getting drilled in excruciatingly painful correct grammar, take careful notice that 95% of your class are women. While enrolled, I found smoking hot single young co-eds that needed a study buddy.

On the whole, those women tend to think they're going to have an easy middle class life as an English teacher in public school. Once they get that job, most of them discover how terribly wrong they were. They last for a few years, or one case a day, and moan "I got to get myself another job."

Disclaimer: dirty old man with a bachelor's degree in English, class of '03. At the time, I showed off the University of West Florida campus to a romantic interest that kept wondering why all these women kept rushing up to us, telling her what a great guy I was. That was because I shared my typed classroom notes with them.

I'm only mentioning this because I've was a newspaper editor prior to going back to school for a degree in English. Us professional journals can take detailed notes to share with classmates.

What happened to my post?

In the uk I often see ads for proofreading qualifications, me of these is: www.wbproofreading.com
Might be worth having a look. Working full time; I’ve often been interested, but don’t have the time.
They have a note to quote ref: SZ120P in case you need it, this ad was in, ‘The Big Issue’.

Also, they're going to want to pay you peanuts. But if the allergens don't bother you, give it a try. An in-house job would be better than freelance.

it depends on whether you're talking hard copy or online. As a 55-year member of the working press, I can tell you that worthwhile hard copy editing or writing jobs are going the way of the dinosaur. Everything seems to be online. You don't need a degree in English. If you're intelligent and well-read you'll do fine. If you have a doubt about a word or phrase, possible slang, new invention, there are many sources to look to. Don't expect to make a fortune. Let us know what you find. If you want to talk, PM me a contact number and I'll call.

Without knowing a bit more about you, it's difficult to advise fully. Have you only ever lived in the USA, for instance, or are you familiar with the culture and slang of other English speaking countries?
As a publisher of English language magazines in Spain, I was always open to taking on a part-time proof readers.
Qualifications did not matter. I have come across far too many "qualified" people who did not understand grammar and punctuation. I looked for well educated people who could write well and pick up mistakes. After all, a spelling checker can highlight most typing errors.
Try contacting provincial publications and book publishers, rather than National papers.
Also, people wishing to self publish with Amazon and their ilk.