A treatment is basically a shorter version of a screenplay which describes all the scenes and interactions in the film. There is no dialogue written in treatments. If you must mention a particular interaction between two characters, then you just describe what they’re talking about. The treatment writing process may involve researching the topic of the story too, especially if it’s based on real events.
Treatments create an outline for the full screenplay. A writer might give a treatment to a director or producer to see if they’re interested in the story. If it meets their approval, the writer will be told to write the entire screenplay. The length of a full screenplay is typically between 90 and 120 pages. Each page is equal to 1 minute of film time on average.
There are screenwriting programs like Final Draft which make screenwriting easier. It formats the slug lines, action, and dialogue for you. This will be very convenient during the screenwriting process because it allows you to focus more on the story and less on the menial formatting tasks.
Once your screenplay is finished, you can expect to receive strong criticism and feedback from whoever is reviewing it. This could be an agent, director, producer, actor, studio executive, another screenwriter, etc. Based on their comments, you will likely be expected to write another draft of the screenplay. Sometimes it may take 4 or 5 drafts before the script is considered good.
In most cases, you are trying to write a screenplay because you’re hoping either a film studio or independent filmmaker will purchase your script and turn it into a film. This is difficult to achieve because studio executives receive dozens of screenplays every day from writers just like you. They don’t have time to read them all.
For this reason, you need to write a query letter to the executive before you send the screenplay. This is a formal letter which introduces your story idea and hopefully gets them excited about it. If they reply to your letter with a request to read your screenplay, then you will mail the script to them. If the script impresses them enough, it could lead to a deal where they purchase the script from you.