Read the book a few times. Before turning one carefully thought out medium into another, you need to carefully think. Be able to recap everything that happens in detail.
Using 3-by-5 cards, write the details of each scene on a separate card.
On more 3x5 cards, write down interesting lines of dialogue written in the book. Though you will add much more dialogue, there are usually lines in a book that need to be heard on-screen.
Remove any scene that does not need to be there. Books deal with plot on multiple levels. Choose one or two levels and remove anything not dealing with them.
Decide if you want a straight adaptation, or a loose one. Loose ones refer to all of the modern movies that have been based off of a Shakespeare play.
Feel free to shift some points around or change motives. This is an adaptation and what happens in a book may need to be altered to make sense on a movie screen.
Sit down and write the script.