There's a neat series of snippet quotes from professional screenwriters about the craft on BuzzFeed.com. How ideas are born obviously is a natural and organic thing that is personal to each writer. But how you approach taking that idea and crafting a story involves so much more and these folks are worth the time to listen to with regard to how they write from idea to first draft. 

From their site:

BuzzFeed spoke with some of the industry’s top writers and directors to learn how they develop a tiny germ of an idea into award-winning screenplay. They discussed everything from how they get started, to how to sit down and write, and how to balance dialogue and structure.

Here are some samples:

Richard Linklater (SLACKERS, BOYHOOD): There are a million ideas in a world of stories. Humans are storytelling animals. Everything’s a story, everyone’s got stories, we’re perceiving stories, we’re interested in stories. So to me, the big nut to crack is to how to tell a story, what’s the right way to tell a particular story. So I’m much more interested in narrative construction.

Jeff Nichols (MUD): I started thinking about Mud in college. [Nichols is now 34.] I’m a very slow writer, and the typing, which most people consider writing, that’s a very last step for me. I heavily outline things. Even before I write anything down, I think about things for a really long time. It’s like a tape ball that you just add detail to, and that’s what happened in this case.

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And here's a great video from BAFTA that includes other professional writers talking about how they start writing from idea to first draft: