How do I add a banner?

[allyndra]

Oh, wise and knowledgeable flist, share with me the ways of the livejournal. Everbody has such pretty banners, and I want one, too. But I have no idea how to add one, and if it tells me how on th FAQs, I managed to miss it. Do you have to have a certain layout? I've found a couple banners I might want to snag, should I figure this out.

So, help me, flist. You're my only hope.