Explain your story idea in 25 words or less. In film jargon, this is called a 'log line' or 'tag line' and its purpose is to provide a hint of what the movie is about.
Examples:
"An actor's resemblance to a recently deceased dictator lands him the role of a lifetime." (Moon Over Parador)
"One ordinary couple; one little white lie." (Date Night)
"A story about love at second sight." (While You Were Sleeping)
"Heroes aren't born; they're built." (Iron Man)
Identify the primary genre of your story idea. Examples of this include comedy, drama, horror, fantasy, romance, thriller, western, and science fiction. Although movies often incorporate elements from multiple genres, the guiding principle is to envision what section the film would be found in if someone were trolling the aisles at the video store. Along with the genre, define your film's target demographic. Examples include families, teens, females, mature adults.
Make a list of your movie's main characters as well as the supporting players whose actions contribute positively or negatively toward the resolution of the plot. A solid movie idea must include a protagonist, an antagonist, and a sustainable conflict or quest that is driven by reward, revenge or escape and which will require the hero to take escalating risks.
Take into consideration the budget and resources of the film director. These have a direct bearing on the size of the cast, the number of different settings, and even whether the plot unfolds in a contemporary timeframe or a historic venue calling for period costumes, props, vehicles, etc. Keep in mind that a lean, economical script is more appealing to a fledgling director than one that carries a big price tag.
Adhere to the rules of three-act structure by giving your film idea a beginning, a middle and an end. For a one page synopsis, each act is allocated one paragraph of description. The content should not only introduce your characters as they make their appearances in the story, but identify the major plot points and revelations that move the action forward. You may find it helpful to create a storyboard first with index cards to determine the best and most logical order for events to unfold.