You can find all the details on her blog here.
Essentially, you sign up for the swap with your postage details and little bit of information to help your Secret Santa. (You need to sign up by 18th November.)
From Dewey's blog:
What you have to do next is send that person a little something - it can be a book, a journal or bookmark, a box of holiday cookies, a mixed CD, whatever you can think of. It doesn’t have to be anything pricey, of course. Second hand books are perfectly acceptable, as are homemade gifts.
A different person will be assigned as your Secret Santa, and you’ll only find out who they are when you get their package in the mail.
Something to keep in mind: Because there are book bloggers from all over the world, this is going to be an international swap. I understand that not everyone can afford to send a package overseas, though, so if that’s the case with you, please don’t feel that you can’t sign up. Just include a note saying so in your e-mail, and we’ll make sure you get a blogger who’s near you.