This is the process I take when creating a movie script. There is no one right way to create art. There are numerous books and classes on how to write a script, the best one is the one that works for you. Hopefully you will find these steps helpful. They are a hybrid of the different schools of thought on how to write screenplays.
1. Concept
2. Title
3. Three Acts
4. Characters
5. Subplots
6. Scene Diagram
7. Beat Sheet
8. Treatment
9. Movie Script
10. Rewrite Movie Script