I blogged recently about my process for kicking off a book. I'm into writing it now (chapter 3 done, 12 more to go). As others have noted, there's more to it than just figuring out the basic plot points. Here're a few other things that are involved.
First is The Book. Every story of mine has a special notebook, usually something picked up at random while browsing in the office supply store. Not a spiral book per se, but something small, portable, and well, "right" for writing.
I create sections with those marvelous sticky tabs: Who, Where, Timeline, Snippets, Misc. In this book, "Misc" contains bits about the book on which I am basing this one (it's a Remembered Classics book, so I need some details from the original book). Then it's on to details.
Characters: I need to have pictures of people in my books. I always use actors and then describe them in rather broad terms so their identity is somewhat unknown. I find pictures of them and keep them in The Book. I jot down info about them: age, height, weight, eye color, nervous trait, favorite startled expression ("Holy crap!" "Damn", etc.) I do this for the major players in the book, villain and hero alike.
Where: oh, I love this part. I look at house plans and find the perfect houses for my people. This book takes place in a mansion and I had a firm idea in my mind of the floor plan. So I had to go online and skim through various plans until I found the right one. Then I skimmed through assorted designer sites for furniture, etc. I've designed so many lovely homes this way.
Snippets: these are the scenes that pop into my head when the characters start to speak in my brain. Sometimes this is just a cryptic note or two ("Charlie sees stray dog; approaches; John acts like she's crazy; contrast: she trusts; he doesn't"). Fuller scenes goes into the Snip file on my computer.
Timeline: I keep two types of timelines. Every character has a back story, so I'll jot down what their timeline is ("Charlie married when she was 22; her husband died when she was 30; she retired at age 50").
And I also write out the timeline for the story as I write it with one or two points from the chapter ("Chapter 1: August 15/Sunday night; sticky humid day: Charlies agrees to interview for job. John comes to her house. She insists on taking her car to the mansion").
The timeline bit is really important because I need to track the days, the weather, etc. so there's continuity from chapter to chapter. I normally write a book from start to finish, but I do get interrupted now and then and it's good to have those reminders.
So that's the setup for the writing. Now it's just a matter of getting the story that's in my head down on paper. Piece of cake, right?