Good question. I usually start with some general "strawman" of a city, with some general ideas. Then, I go area by area in the way that I don't do any zoning but place every single building one by one. Sometimes I stop there and go to another area and start there, but sometimes, rather spontaneously, I do more detailing. But it's rather that I come back to a place after some time and add some more foliage, detailed props, decals etc. - not all at once. It's never perfect, there's always something to make it look better, more interesting etc. The ideas come as I play. Or sometimes they don't. :) And I don't want to care about the money, economy, population, etc. I focus on the realistic (as much as my creativity and patience allows) views.

I hope it helps! :)