Not all books need world building. Only two of my story bibles have sections for this—my vampire story, and the spec fic I’m doing for NaNo. Contemporary doesn’t need it, but if there are any rules in your world that are different from real life, make sure you have one. Or if you’re writing something with intrigue or mystery or something where you need a timeline…
Oh, fuck it. Leave space for world building anyway ;)
So, I break my world building section up into stuff like the rules of your world. How vampires are made, or what happened when the clock ticked over into 2000, or the history of the world (there’s generally something pretty huge way back that’s brought the story into being in my brain), and then I just write down whatever has made itself clear so far. Paragraphs, bullet points, whatever.
One thing I don’t do—and I keep repeating myself on this point—is force it. Let the rules and history and the canon of your world develop in your brain before you go squeezing it out on paper. Think about it. Talk about it, to anyone who will listen and feedback and discuss without telling you you’re mad. Avoid those people ;)
Once your world becomes real in your mind you can write it down. If it’s real to
you me, it seems to make sense and I don’t have to do as much retconning after the first draft is finished.
And I’ll do almost anything to avoid the dreaded edits O.O