If you are writing from scratch then there are established industry stages of screenplay formation which you should know about; if only because everyone else uses them. If you are writing your script in your own time and your own way then you can use these steps at any time they seem appropriate.
Log line
A log-line is a single line summarising the entire script or story. It should be dynamic and pack a punch for listeners. This is the ‘big concept’ moment. If someone likes this first pitch they’re likely to want to hear more so make sure it has the ‘wow’ factor.
Synopsis
Summarising the story in present tense is the purpose of the synopsis. It consists of anything between one paragraph and one or two pages. It is good practice to have different versions available, not only for pitching to others but also for yourself; there’s nothing better than being able to rattle off different length versions of your own story without the ‘ums’, ‘errs’ and ‘well it’s kinda like this’.
Step-outline
Here, your story is told in a series of steps. You describe each scene and its action in one or two sentences. Step-outlines are terrific for building up the stages of your story without yet having to fill in all the details. You can then shuffle your short scenes around endlessly as you progress.
Step-outlines are often written on index cards which gives you the advantage of being able to lay them out in front of you and re-arrange scenes at will as new scenes are created. This physical accessibility is often an extremely helpful and freeing tool for the writer at the point when the monitor screen has become an unforgiving and unimpressed tyrant.
Robert McKee, in his classic book ‘Story’, states that ‘the writer never shows his step-outline to people because it’s a tool, too cryptic for anyone but the writer to follow’ (Story, Methuen 1999, pg.413). Once you have the entire script in this format it is a great foundation from which to pitch your story to any interested parties.
Treatment
Every scene is developed into paragraphs and told as if it is happening in the present tense. The action is expanded and described in more detail than the one or two sentence step-outline but only what the audience would see and hear. Dialogue is summarised or excluded, except for occasional important lines.
A treatment will generally be between ten and twenty pages and is often used as a tool for raising financial baking and/or interest in the future film.
If you are ambitious and thorough, a treatment can be anything up to one hundred pages of fully fleshed out story, covering all the elements of the plot or idea. This is a short story format, almost the beginnings of a novel. If you go down this route, you can bet that any script developed from a long treatment will be a good deal more solid, coherent and thought-through.