Becoming a writer...

How difficult is it to become a professional writer? I don't mean just fiction/non-fiction either, I mean in any field- technical, grant writer, correspondence, etc. (Though I can imagine it is easier finding jobs writing for toaster oven instruction manuals than writing avant garde lit). Do you need to work through an organization? Is there freelance work? What sort of qualifications do you need, or body of work prepared to show a potential employer?

You write something, and if its very very good and you are lucky enough to get a publisher interested, they might give you an advance and put out a print run.  If you have more work to show, they might even give you a multi-book contract.  If by some chance people take notice and the books sell, the book might get new print runs, or get picked up by foreign publishers. If it sells really well you can make royalties, and the publisher will push your work harder.  All this happens for about 0.0001% of people who want to be writers. 

If you are really good, or at least think you are, and have some finished works you want to get published, you need to get an agent.  Its almost impossible to get something through without an agent.  You will almost never need qualifications; its about having finished, quality work to show publishers, and about having connections. 

Finish something.  Find out different publisher's guidelines and send them to them.  Wait 3 months to a year for replies.  Collect rejection slips, probably for a long time.  Keep revising and keep writing.  Go to conventions and talk with writers and publishers.  Be aggressive.  If you think you're amazing and have some money, get an agent.

Technical writers for things like manuals and shit are an entirely different manner, they're employed by the company that makes the shit, not a publisher.