Freelance copy editing isn't just a simpler offshoot of freelance writing in general, but an important discipline in its own right -- and a rewarding one.
To become a successful copy editor you need to know how to use style guides. With some exceptions, editors of newspapers, magazines, and other print publications require you to write in a homogeneous style, both to compensate for writers with occasionally sloppy spelling and usage and to ensure consistent terminology over time. (This is important with newspapers: the names of foreign leaders, organizations, and other foreign-language nouns are often subject to variant spellings.)
The most commonly used style guides include AP (Associated Press), MLA (Modern Language Association), and Chicago. Any budding freelance copy editor would do well to own a copy of each of these, and to become familiar with their use before applying for jobs. Prospective employers will not hire copy editors who lack knowledge of style guides. Use a product like StyleEase software to help with style.
Fact checking is another prime skill for copy editors, as it is a publication's first line of defense against accusations of libel or misrepresentation. Fact-checking is a simple procedure: call the author of the article, ask for his or her sources, and, if warranted, call the sources directly to confirm quotes or statistics. Different publications will have different procedures for fact-checking, all of which should be explained when you take a job.
Beyond that, all that it takes to become a successful copy editor is a sensitivity to cumbersome phrasing, grammar, and spelling, as well as a sensitivity to an author's personal style. Many novice copy editors take a far too forceful approach to their work, effectively rewriting a reporter or other writer's article for them in line with style guides and their own ideas about what makes good writing. This isn't the function of a copy editor. Yes, clarity, grammar, and other issues with writing mechanics are all important, but a writer's ego is important as well, and a too-free hand in the editing process can alienate a publication's staff reporters and foster general enmity.
Since rewriting someone's article causes you more additional work as well, why would you want to do it? Instead, just try to achieve sufficient clarity while leaving as much of the original article "as-is" as you can. If there are any substantial portions of text that inhibit clarity or exhibit serious mechanical errors, talk to the writer personally before making any changes. Yes, it’s an extra step, but one that ensures professional respect in the workplace.
If you don't want to work for a publication, there are plenty of opportunities available for freelance copy editing, both for corporations and for private individuals. Educational publications, in particular, are always looking for good copy editors, and book publishers and literary journals always have a few spots available. You can find out about these opportunities through classified ads, or by making inquiries directly to the company. There's typically a lot of competition in these sorts of jobs, so a solid interview technique and some excellent samples are mandatory for securing work. Once you have your foot in the door, though, corporate copy editing can provide a stable -- if occasionally dull -- source of income.
Copy editing projects offered by individuals are another option, and one which can bring you a more varied body of work and a much more informal attitude toward style guides and format restriction. But this option carries with it some heavy caveats. Often, copy editing projects given by individuals amount to ghostwriting without appropriate compensation, and pay rates can be sketchy as well, ranging from low to nonexistent (with a promise of "resume experience," maybe.) Although when work is consistent, low pay isn't necessarily a problem, individuals can rarely guarantee a sufficient volume of work to ensure your livelihood and a decent career.
Before you accept individual copy editing projects, make sure that you know how much you'll need to make per hour to make the project financially worthwhile (as well as an estimate on how many hours the project will take), and don't accept less than that hourly rate. You may get less work with this approach, but clients won’t rip you off either -- an important consideration for professional copy editors.
Copy editing is a good, low-stress writing job, enjoyable on its own merits or as practice for other freelance writing goals down the line. You can succeed as a freelance copy editor if you familiarize yourself with style guides, and have a good grasp of grammar, spelling, and style usage.