Writing a screenplay based on a book will not be an easy task. You will have to make cuts and changes that will upset some of the people who love the book. The internal conflicts the characters in the book go through must now be externalized. The settings in the book will have to change to make it more cinematically friendly. You might even have to change some details about the characters, in order, for the script to become more visual. You must remember to stay true to the heart and soul of the text for the script to be creative and inspiring.
The first thing you need to do is obtain the rights to use the material. If the book is not your own, it is necessary to contact the author or publisher of the book before you write the script. While you can make a great screenplay, no one will ever see it, if you do not have the rights. You can contact most authors through a website.
Once you have secured the rights to create the script, you must read the novel twice. The first read through should be for fun. After reading the book, if you are not inspired or do not feel passionate about the story, then do not progress. If you feel passionate about the story being told, then read it a second time. While you are reading it, imagine the text as a screenplay. You should picture the scenes in your head. This will give you a clear ideal as to what scenes have a strong visual image and what scenes need to be dropped.
Analyze the characters. Doing this allows you to combine, eliminate, and adapt the characters. Do not be scared of this step. There may be two characters that are so alike you can combine them into one person to create a better script. There may be fringe characters that do not add to the story. If that is the case, then cut them out completely. You can even take the time to adapt the leading characters to create a theme. You should make the changes you need to make to develop the characters.
You will need to make the story more visual. This is one of the most important aspects of developing a book into a script. You will need to look at the internal conflicts the characters are going through and develop them to manifest in an external form. Take the time to examine each scene and setting. You should look to see if there is a lapse in visual content and make the changes necessary to create a more cinematic story.
Writer the script based on what the story is telling you. Preserving the major scenes and important sub plots will help you keep the heart and soul of the book. You do not want to stray too far from the story, because this is what the audience has come to know and love.
Finally, after the script is finished, then you should compare it with the book. You will need to make sure that there is a strong relation between the two texts. If you find that the script is complete different than the book, then you need revise the script.