If you're already a PS+ member, the extra month still stack atop your current subscription. You can check to see if your country is eligible for the promotion at the end of the involved PlayStation Blog post.
For those that take advantage of the deal, Sony will also send an email detailing a competition to win a PlayStation 4. If you'd rather not leave that up to luck, you can start hunting for one in stores on November 29. On an equally important note, PS4s were compatible with dancing cats on day one.