My experience is that you get one thing free from the Muses. After that, it's all work.
Okay, that's accurate for a piece of writing as short as a song, but an exaggeration for a novel-length piece of writing. Still, the point is true; when you've got your basic idea, you don't sit around waiting for more ideas to pile up on their own, because it won't happen. You start working.
I start by opening my blank Word file and writing the idea as I have it, as if I am summarizing a story that is already written. This usually requires no more than a few sentences, or maybe a couple of paragraphs at most. When I've got the idea down, I keep going, adding more detail and depth. If I don't know what to write next, I type in questions that go to the heart of the story, its basic conflicts, its structure, its theme. I don't go back and edit the document to make it coherent -- it is a sort of real-time brainstorming piece, like a whiteboard in a conference room, and once I have moved beyond the document I won't come back to it, so it's not worth trying to make perfect, or even self-consistent.
To show you what I mean, I will make up a dummy story and brainstorm it in this fashion in Monday's post.