Create an outline of your story, highlighting the most crucial elements. Introduce the main characters, plot, climax and conclusion. Do not leave out the ending, as the reader will need to know the entire story.
Write a your first draft of the treatment from this outline in first or third-person. Worry more about explaining the story at this point rather than word count or length. You can whittle it down later. However, do try to keep it at a manageable level.
Edit the first draft by removing any unnecessary sentences, flowery language, adjectives, adverbs, etc. If it isn't pertinent to the progress of explaining the story to the reader, remove it. The editor or producer is pressed for time, and are more likely to read shorter treatments than longer ones.
Set the treatment down for a day and then read it with fresh eyes. Again, remove any unnecessary flowery language, adjectives, adverbs and so on. Get to the meat of the story.
Research a market for your story using online resources or writer's magazines and books. Send them the treatment in a 9" x 12" manila envelope so as not to fold the treatment. Also include a cover letter and a self-addressed stamped envelope for the reply.
Register your treatment with the Writer's Guild of America. It costs $20 and will protect your story from being unlawfully copied or otherwise stolen.