Secure the right to use the material. Before you can begin to write your script, you will need to approach the author or book publisher for permission to use the text. If you do not have the rights to the material, you could produce the best adaptation in the world but nobody would ever see it.
Read the novel twice. The first read through should be for fun. If you read the book and do not feel inspired or passionate about the story then there is no point progressing. During the second read through you should imagine the text as a screenplay. Picture the scenes in your head to see which bits have a strong visual image and which should be dropped.
Combine, eliminate and adapt the characters. You should not be scared to change the characters. Two characters may be similar, so, to strengthen the script, they could be combined into one. If you feel that fringe characters offer little in developing the story, cut them out completely. You may even feel the need to adapt leading characters to develop a theme or motif. Make the changes that you need to make.
Make the story more visual. One of the most important aspects of converting a book into a script is making it more visual. You should look at internal conflicts and allow them to manifest in an external form. Examine each setting and scene looking for lapses in visual content, and make changes to create a more cinematic story.
Write what the story is telling you. Keep the heart and soul of the story, preserving major scenes and important sub plots. After all, the audience has come to know and love the book for what it was originally.
Compare your final draft with the book. Can you see a strong relation between the two texts, or is your script unrecognizable alongside the book? If the two are worlds apart, you should revise your script and work on a new draft.