First, let’s distinguish between copyediting and proofreading. By most definitions:
Copyediting = working on the raw manuscript, ensuring consistent and correct spelling, grammar, syntax, and style, while maintaining the author’s voice. Checking numbered and alphabetized material for correct order. Checking chapter heads against the table of contents. Ensuring that the text, including footnotes, bibliographies, and other elements, conform to the specified style (whether Chicago, APA, a house style, or some combination) and/or dictionary. Often simple fact checking (spelling of proper names, dates) and verifying URLs. And a bunch of other mechanical editorial tasks, depending on level of edit. It is the final mechanical edit before the manuscript goes to the typesetter.
Proofreading = comparing that copyedited manuscript against the page proofs that were set from it. (Very rarely, if ever, does a person proofread copy that he or she copyedited. This is a fresh set of eyes.) You are checking to make sure that the text was set as marked. These are the pages that will go into print! You are also the copyeditor’s second set of eyes, picking up any editorial errors that he or she may have missed – but at this point most clients do not want you to be making massive editorial alterations (EAs), but rather query any major inconsistencies. The other half of the task is checking that the material was set according to the design specs: correct fonts, colors, spacing, tints, rules, placement of art, margins, page depth, headers/footers in place. Get out your pica measure. You’re probably going to verify that words are hyphenated correctly, that there are no stacks (same words at the beginning/end of successive lines), rivers (of vertical white space between words), loose/tight wordspacing, or bad kerning. Are all chapters supposed to start on a recto? Are all the dingbats where they belong? Are all the H1s set as H1s, the H2s as H2s, and so on? In tables, do columns of figures align correctly? Do numbered lists clear for 10 (allow extra space before 1-9, so the periods align with 10 and up)?
It ain’t always glamorous. Right now one of my ongoing projects is proofreading a physics textbook. That includes verifying that the Greek letters are italic or roman as appropriate, math is set properly, equation numbers and tints are in place (they’re easy to miss), and proofreading some rather complex art that was set from the author’s very messy drawings, following some 16 pages of art specs as a guide. (Is this a velocity vector or an acceleration vector? The colors are different.) Is the italic v for velocity the pointed or rounded style? Which type of epsilon are we using? Are fractions builtup or inline? Are multiple exponents aligned, or is the second one a little higher? Is there a hair space or no space between a trig function and its argument?
Mind you, I enjoy proofreading, but it can get quite complex. I’m not just toodling along picking out typos. And the language needs to stay the way it is unless there’s an egregious error – that’s not my job.
Even when copyediting, you seldom get free rein, especially in fiction. Right now I’m copyediting a novel by a best-selling author (no, I’m not telling who). She has enough cachet with the publisher that I’m instructed to leave her informal style alone unless it’s out-and-out incorrect or the meaning is unclear. Tough waters to navigate. I don’t get to slash and burn like I’m her English teacher. And for a book-length project, you need to keep your approach consistent from beginning to end, through hundreds of pages. That can be tricky, especially when (as is happening more and more) the project comes to you in batches and when you’re working on Chapter 9 you find something that was done differently in Chapter 2, and you’d rather do it this way, too bad because you sent Chapter 2 back three weeks ago and you’ll never see it again.
And yeah, it’s not all sexy reading. I just finished a book in the financial field that was an absolute snore. But the money was good. I do a fair amount of math and science, which some editors won’t touch.
As for finding work, good luck (and I mean that sincerely, not sarcastically). It’s a tight market right now, and as more companies downsize and dump their employees into the freelance pool, it’s gonna get tighter. But there’s lots of work for the right people. I started out with no publishing experience and went right into freelancing, which I’m told is pretty rare. I started with Literary Market Place (available online or at your public library reference desk) and peppered my targeted potential clients with resumes. That first mailing of 40 resumes (followed up by phone calls) netted me a couple of tests and one client (after about six months). I’m told that’s an amazingly good response. Guess I got lucky. You’re going to want to research the market, acquire the appropriate reference books, and polish your skills – which need to be more than catching typos, but being able to do all the things I named above and more, and darn near error-free. That’s what clients will be looking for.
Googling “freelance copyediting” and “freelance proofreading” should also turn up more resources than you can shake a stick at. Mind, as newbies ask time and again, there are no “web sites where I can sign up for freelance jobs.” It’s just like regular job hunting: you have to sniff out who is using the services you’re offering, and then sell yourself and get them to hire you. Plan on taking their in-house tests, especially if you have no professional experience.
Also, everything that tiltypig said, with one exception: I often find myself making pretty decent money when I’m working for project rates (by the project or page, rather than hourly). At least for getting to work in my jammies.