Acquire the permission from the copyright owner or the screen rights owner of the novel you wish to turn into a screenplay. This is the first crucial step. Without acquiring the right to turn a novel into a script, you risk investing a lot of time writing something you may never be able to use. The author may not own film rights. First contact the publisher and see if it owns the film rights. If not, contact the author of the book or the author's agent. You will have to negotiate for either license rights or assignment rights (see resources).
Commit the novel to memory so you can determine what will need to rewritten to adhere to the screenplay method of telling a story. Many novels contain internal writing such as character thoughts that can't be easily translated to a motion picture. You will need to find visual ways to dramatize those scenes.
Determine what to leave in and what to omit. A movie is approximately two hours long. A screenplay runs about 120 pages. Fitting a novel between 500 and 900 words into a script means you'll need to find the key plot points in the novel and focus on those elements to tell the story. Novels often go off in directions that may be unnecessary to telling the story on the big screen. You want to capture the essence of the novel in your screenplay. Think in terms of boiling the story down" to its essential elements.
Consider rewriting scenes from the novel. Sometimes it's impossible to tell a story onscreen the way it's written in a novel. You should be open to writing something original to fill story gaps or to accomplish the same story goal as the novel. This may even include telling the story in a different order than the novel tells the story.