Having done a module on editing in my Bachelor's and an editing project as a dissertation equivalent, proof-read a few friends' dissertations and picked up a couple of other odd proof-reading jobs I've decided that it's really quite fun so, as I have some free time during the day, I'm looking at doing a bit more.
I'm a native English speaker living in France so imagine I could pick up a fair bit of work correcting dodgy ESL grammar but I have no idea what that's actually called (am I proofing or editing?) and whether it's viable or not. Is it worth doing a course to make sure I'm doing this properly? If so, what's a good course to do? Do I need to do a post-grad or are there short courses? How does one advertise oneself?
Also what kind of rates should I be charging? I charged about ?5/1000 words but that was for struggling, penniless students and I remember all too well how that feels!
Copyediting: checking text that hasn't been flowed into book / page layout and this entails checking grammar, spelling, style, consistency, etc. It might also entail inserting tagging / styles for a typesetter.
Proofreading: checking text that has been flowed into book / page layout (hence reading book 'proofs'). It entails checking that the typesetter has made all the copyediting corrections, checking page layout, checking the typesetter has followed the page design, as well as a general reading for sense.
It sounds like you have been doing copyediting. I suggest you consult Society of Freelance Editors and Proofreaders for info about getting work and how much to charge (they have a website). I don't do much proofreading now but when I do I charge between £12-£14 per hour depending on the work.