Um...
Worldbuilding does not have any rules except this one: make your world rich.
Not money-wise, of course, but make it deep. Know your world inside out. Make it real- doesn't matter if it's full of magical creatures.
Now, of course, the highly advisable guild lines include creating limitations and laws, understanding the economy, the politics, the culture, having RULES- in other words, making your world a real place, deeper than just what happens in the plot.
For really good Worldbuilding advice, go to Holly Lisle's website (just google it) and look up her articles.
But the most important thing about Worldbuilding is that, if you never actually write the story, it's useless. So don't spend too long on it. It's important, but not as important as the writing.