Watch the film and identify the principal character and the main plot thread. If the movie hasn't been made, write out the expected storyline in as much detail as you can. You want to identify the most important aspects of the film to highlight in your synopsis.
Think about the audience for your synopsis or idea. Who reads it will have a great bearing on how you write it. For example, if you intend it as a pitch to a movie studio, you need to know which demographic they might want to market it to (tweener girls, older men, etc.) and what kind of genre the film might be. If you're summarizing a movie for a newspaper or an academic source, you probably want to keep any subjective opinions to a minimum and just report the facts. If you have that idea clearly in mind, it will help guide your writing as you develop it.
Write down a one-sentence synopsis of the film. It should be concise and to-the-point, serving as the main thrust of your write-up. For example, "swashbuckling archeologist battles Nazis while seeking an ancient artifact" or "A cyclone carries a Kansas farm girl to a magical land, where she must seek the help of a mysterious wizard to get back home."
Flesh out the one-sentence synopsis into a lengthier synopsis or idea. The exact length depends on the parameters you're adhering to. In some cases, it might be just a single paragraph. In other cases, it can adopt a five-paragraph format or something even longer. Regardless of the length, you need to include the entirety of the film's story ---- the beginning, middle and end ---- as well as key thematic elements such as "the hero learns the value of others" or "the filmmakers comment on the evils of racism."
Let the synopsis or idea sit for a day or so if you can in order to gain some perspective.
Revise your synopsis, correcting any grammatical errors and tighten up the language to smooth out the rough patches. With this style of writing, you probably need to make cuts such as streamlining the synopsis and removing anything you don't need. The key is to make those cuts while still keeping the essential elements intact. That might take a little practice. If you have a friend whom you trust, ask him to go over the synopsis and make helpful suggestions. A more objective pair of eyes might be able to catch mistakes and suggest improvements.
Finalize your synopsis and print it out, following any other specific instructions your assignment might require. For instance, if you're submitting a movie idea to a studio, they might require a cover letter detailing your contact information. Or, if the synopsis is part of an academic assignment, you'll need to put the name of your class and professor in the upper right hand corner of the page.