Know what a treatment is. Boil down any highly visualized screenplay into its basic components, and what you are left with is essentially a short story. A short story is exactly what a treatment should resemble. There are no hard-and-fast rules regarding their length: Some treatments are short as one page, while others may exceed 25 pages. The best approach is to create three treatments of varying length, with each containing more details. The tone of the prose should ideally match the tone of the screenplay; if you were writing a romantic comedy, you would use a lighter style than if you were writing film noir.
Write the treatment in the present tense, with as much verve and style as if you were actually writing a short story. In other words, instead of writing, "Jack Torrance picked up an axe and smashed through the door to yell `Here's Johnny,'" you should write "Jack Torrance picks up an axe, smashes a hole through the hotel door just large enough to stick his head through, and announces his presence with a hearty, `Here's Johnny!'"
Format the treatment into paragraphs, like a novel, rather than the strictly regimented formatting required of the actual screenplay. Do not indent the first line of each paragraph, but align with left justification. At the end of each paragraph, insert an extra space. The treatment should also be single-spaced.
Observe the difference between a treatment and a synopsis. A synopsis is a simple recounting of the plot and major highlights of the screenplay. A treatment is far more expansive, and should include much more information, as well as be far more entertaining to read. The best way to distinguish between a synopsis and a treatment is that a synopsis is intended to tell what happens in your screenplay, but a treatment is designed to sell the screenplay. When writing a treatment, keep in mind that, in addition to getting across what the story is about, you need to write it with the intention of making it as salable as possible.