How do I become a web design freelancer?

I did a couple of "unofficial" (read: under the table) freelance web design gigs when I was unemployed. Now I have a new full-time job, but it's not in the web design field. And I think it would be a nice way to earn some extra cash on occassion.

How would I go about being officially recognized as a freelance web designer? Would I need to essentially start a business to get a vendor number, or something like that?

Please, only answer if you actually have a clue about this sort of thing.

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