Well, I start out with a one sentence pitch to myself, to see if the concept sounds viable:
"Two downtrodden girls escape from tyranny into a world of magic."
From there I come up with a short summary, anywhere from a few paragraphs to about two pages. This sets up character names, major conflicts (in my case, the conflict is not actually getting to the magical world, it's what happens once they get there), etc.
For me, that got the basics in place, then I had to start world building and setting up rules. Now I'm working on a chapter-by-chapter outline before I actually sit down and start writing the book.
Dunno if that helps, just some thoughts.