I find a good place to start when designing a world is to get a clear over all picture of what you want to accomplish. I tend towards brainstorming a list of phrases, adjectives, and influences I'd like to see in the final creation. I use this as a reference at all steps of the process as this helps to keep the idea more consistent and true to the original vision.
The next step I take is to solidify this list into a basic summary. First sentence is a teaser or summary of the entire world. The next few sentences are the items that are not congruous or need to be featured. I always finish the initial paragraph with an element that is out of place or doesn't seem to match the setting. I will use this as inspiration to make the world more unique and interesting.