Think of a title. A good title is something that is engaging and interesting the first time you read it, but after finishing the story, shows a new depth of thought, or possibly invites different interpretations of the material. A title is extremely important for a treatment because it's the first thing any readers will see.
Write the logline. A logline is one or two sentences (usually one) that describes the basic conflict of your story. A good logline also indicates theme and sometimes even basic character. It should give the reader a hint as to whether the story will be a comedy, a thriller, a drama and so on. Many producers are extremely busy and may use the logline in concert with the title to decide if the treatment is even worth reading.
Break your story into three acts. In simplest terms this is merely: beginning, where the characters and conflict is established; middle, where the stakes rise and the conflict takes new twists; and ending, where the conflict reaches a climax before being ultimately resolved for better or worse. Briefly outline your treatment in these three acts.
Write the treatment, transferring all major plots and subplots to the page. A treatment's style should be present tense with little or no dialogue. It should read like a short story with great care put into describing character, relationships, story twists and conflicts. Try not to describe any characters physically, except where necessary, as this will greatly limit the director and casting director when choosing actors.
Rewrite the treatment. Like all writing, a treatment needs to be rewritten when finished. Many screenwriters use the treatment as a marketing tool and as such it's extremely important that it be as flawless and engaging as possible.