Research the characters for your script. You need to know who your characters are through and through, everything from age and race to where they went to school and more.
Outline the action in your screenplay. This will give you an idea of the various arcs in your movie before you sit down to write a treatment.
Break each section of your treatment down by scene. Each scene break down should include the following information: whether it is an interior or exterior scene (INT. or EXT.), location of the scene and what time of day the scene takes place.
Include a thorough synopsis of what happens in each scene breakdown. Think of each scene as a mini-story that should have a clear beginning, middle and end.
Write out the scene if it is pivotal to the movie, a great action sequence or contains really excellent dialogue.
Include a title page that has the name of the screenplay, your name and contact information once you have completed the treatment.