First you must find a novel that you would be interested in adapting into film and that you think would make an interesting movie that others would like to watch.
What genre of movie? Would this book make an excellent TV movie, an independent film?
If you think this is an excellent cinematic novel, then read and reread the novel, taking notes on how you might adapt it into a film.
Highlight the important scenes?
What is the POV? Is the movie point of view the same as the novel?
Then reread the book making a scene by scene outline.
The length of a full-length movie is about 120 pages.
Do you have enough material and significant movie scenes for this number of pages?
What about characters?
How many characters are central to the movie? Are these the type of characters who would make interest movie characters? Think about what movie stars could play the characters in the movie? As you write the movie, thinking about these characters help you to write the movie. You can imagine these characters speaking the lines.
Of course, if a production company decides to make the movie, this doesn’t mean these stars are going to play the characters in your movie. Nevertheless, thinking about them helps you to dramatize the scene.
After your first draft is written, reread, take notes, and revise.
Read the dialogue out loud. How does it sound? How would it sound in a movie?
If you don’t know how to format screenplays, then get a sample screen play script and/or purchase screenplay software. You can also find free screenplay software online.
This is just an exercise, of course. If you want to make use of this novel for commercial purposes, you would have to obtain the screenplay rights. You would have to have the permission of the author to adapt their work. You would have to option the rights. You would have to contact the author and/or the author’s literary agent.
If this is your own novel, of course, you would already have the rights to adapt it as a screenplay. These are some of the techniques that you can use.
In addition to adapting novels into screenplays, you can also adapt works of nonfiction. Often works of nonfiction can make more interesting movies than fictional works, especially creative nonfiction that makes use of fictional and cinematic techniques.