How do you write?

Over my Christmas vacation I read John Irving's 2009 novel Last Night in Twisted River. It shows the development of a novelist and the writing process in that it is a story within a story. Interestingly, the career of the novelist in the book is similar to Irving's career. As a writer myself I found Irving's Afterward particularly fascinating. He admits he's gotten some flack over the years, and says of one conversation with a reader,
I was a dinosaur--or worse, a reactionary. ...I had told her a story. But that's what I do. And if you're telling a story --especially to illustrate a point--you'd better know what happens in the story before you start.
This is good advice. :)

Amazingly, Irving starts his works at the end!
Endings not only matter to me; endings are where I begin a novel or a screenplay. If I don't know the ending, I can't begin--and I don't mean that I need to know only what happens. I need to know the tone of voice, and the last sentence (or sentences). I write not only to a moment in time, but to a sound--a feeling. I have to know what that feeeling is, or I won't start.

From the last sentence, I work my way back to where the story begins. This constitutes a kind of road map in reverse. That process--of working my way backward through the plot, from the last sentence to the first--usually takes a year or eighteen months, sometimes longer. But for twelve novels now, the last sentence has always come first. And those last sentences have never changed in the process--not even the punctuation.

Wow!

How do you write?