I won't be telling you how to write a novel in this blog. Maybe some other time. What I'm writing about is moving from your novel to your screenplay. For me, it's simple. After caring for so much detail in a novel, the screenplay is just about the opposite. However, in its pure form, the detail is in the setting up of the scenes. The conflict and creating great characters. The basic form of the screenplay is the scene setting, action and dialogue. More than anything, dialogue is very important. And something else. The ability to create a scene where a lot happens when no words are spoken. An easy one is a Clint Eastwood western where so much is done with few words spoken. You get the idea. When I sit down to write a screenplay, many times it goes fast because I've thought about the scenes in my head for so long and I know how I want to write it. I can sometimes fly down the pages. Still, writing edgy, new, great dialogue is your challenge. And that, yes, can take time. What that means is don't wait until you sit down to write. Jot down bits of dialogue whenever. Use it later. I feel equally comfortable in either novel writing or screenplays.
Just sit down and write.