Hi all,
I'm one of the developers who worked on City Creation. Feel free to ask me any specific questions!
Fred hit the major points; the big key is that City Creation is intended to be highly collaborative. Everyone, players and GM alike, should bring ideas to the setting that they'd like to see in the game, and then you work together to weave them into a coherent whole.
Once that's done you decide what characters you'd like to play. The classic D&D* "A ninja, a pirate, a paladin, and a psychotic necromancer all meet in a bar when a mysterious stranger hits you with a job offer" doesn't work as well in the Dresdenverse - people really need a connection to what's going on around them, so we encourage everyone to get a feel for the world you'll be playing in before you decide what you'll be playing.
I think one of the big challenges with city creation will be limiting the scope of things, especially if the players and GM know the city really well. Two or three big themes and threats are plenty, and sometimes it's hard to stop yourself once you get going!
- Clark
(*That's not to disparage D&D; I love D&D and play it all the time.)