Think. Spend time with your conceptualized story, the characters, what they do, why they exist. Construct your plot line, subplots, and your general storyline. Ask yourself what you want to convey. Think awhile, maybe days or weeks or months. Then write, trying to not think at all, rather just allow the story to unfold as you've conceived it.
Outline a rough chapter plan if you need structure to guide you. You can also write character lists, full bios, back-stories and relationships to other characters. Know why each character is necessary, and omit any that aren't. In novels, you can get away with more characters typically than in a screenplay. But sheer numbers have no value -- characters must each have a purpose to propel the story.
Elaborate on places, settings, descriptions and details that support your story. This is quite the opposite from screenwriting; screenplays must stick to only short, brief scene descriptions and dialogue. Shooting scripts will have camera angles and motion, however, spec scripts do not. The best ones are mostly centered narrow columns on a page with two-thirds of the page in white -- the column is dialogue.
Weave your central characters, especially your protagonist, through the storyline. Key characters will have "arcs" or unfoldings and changes determined by the plot, by interactions with other characters, and by their own inner development. These are the things you will focus on interpreting. On a film, the best screenplay doesn't offer any actor interpretation -- characters do arc, however actors will interpret the details for themselves and their directors and producers.
Eliminate extraneous words. Here, the novel and screenplay are identical. In good writing, every word actually contributes something, and anything excessive is eliminated. Write only what you need to write -- the best details don't require overwriting.
Structure your story from your authentic "voice" rather than putting on airs to try to write like someone famous. Write from where you are, now, with what you've learned and experienced as your foundation. Trust that.
Complete your novel in however long it takes. This can range from six months to 10 years. Don't judge the time it takes you, just write. Once you have a completed draft, set it aside for several weeks. Return to it, reread it, rewrite it. Don't settle for "OK." Go for your best work. Rewrite 20 or 30 times if needed. It doesn't matter. Just finish what you begin.