Snippets of dialogue generally start the process, I then flesh out a scene and if I like it, then an entire story. Sometimes a concept pops into my head that I'd like, and instead of prompting it I try to write it, but again it usually starts with dialogue. I'm not so much for the descriptions, I usually really struggle to describe a scene. That may be a good thing though, because I'm amazed by how often I'm complemented on my attention to detail.