For me, getting the idea part is easy, I have way more ideas than I could possibly ever draw. The overall storyline takes shape naturally from the characters as I pit them against the question I have in mind for them. I generate dozens of variations and choose the one that would make the best story.
Then I plot out that story in a bulleted list, down into a chapter guide that looks like an episode guide for a TV show. After that it's refine, refine, refine, incorporating any better ideas I get and discarding ones that don't work as well as I previously thought.
