Writing a book: Where to start?

Some authors begin a book with a premise to prove (love conquers all, blood is thicker than water). Others have a character who will not let them alone. Some want to write about a situation (a marriage of convenience, a quest for a hidden treasure). Others have an idea for an amazingly intricate plot and want to see it play out on the page.

Me? I've done all of the above. Except for the amazingly intricate plot. My plots, if my books have them, are pretty simple and straightforward, not a lot ...