I just did an isometric exterior for the last Lite Challenge which you can check out. It shows what I started with. Like Gamerprinter says, I used a simple shape outline first (and also built a model in SketchUp for reference) and then started filling stuff in. I'm going to continue working on the buildings here but that will mostly just be painting in Photoshop. I do have an isometric grid set up for Adobe Illustrator and if you want that I will figure out a way to share it with you.

I only posted links to my stuff because I know for sure where they are. You can also search the site for "isometric" or use a Google site search (isometric site: cartographersguild.com) which will get you to a whole bunch of information and ideas on isometric mapping from the guild members who are really good at it.

cheers,
Meshon