Write out the main plot points of the novel and key information about the characters on index cards. Be thorough in your notes. Write detailed character profiles, highlighting important information, such as events in the characters' past that may change how they react to certain situations, or special skills they may have which will be useful later on. Also jot down any key passages of dialogue that might be important.
Pull out the major plot points that are necessary to make a story, and begin arranging them in the standard three-act structure of a screenplay. Act I contains the information the audience needs to know at the start of the story, and the incident that starts the plot in motion. Act II features the difficulties characters have reaching their goals, and Act III is the finale. Then arrange the subplots as needed.
Insert the character information and dialogue you wish to keep as needed into your plot. Remember to show, not tell. If a character has a special skill, include a scene before the inciting incident that demonstrates this skill instead of having the character tell us. Make sure any dialogue you wish to keep fits this criteria.
Write the screenplay using your plot and character notes. Keep the novel handy to consult as you are adapting it. Remember that novelists have to describe things in order to create an image in the reader's mind, which is unnecessary in a screenplay. If you include character description as narration, leave out the physical traits and focus on the narrator's impressions of that character's personality.