Write down your basic idea for the story in three sentences: no more and no less. Think of yourself describing the story to a friend on a bus and your stop is coming up next. Introduce the main characters, what challenge they face to reach their goal, and if they succeed or fail in their mission.
Develop a three-level outline structure from your short description. In point form, introduce the characters, give them a goal, and resolve the story with their success or failure in attaining the goal. This outline acts as a reference when writing the script; it helps the writer get back on track when he wanders too far from the main story he is trying to tell.
Use that three-tier structure to write a short story, or treatment, of the film. Start developing sub-plots for the characters; these are narrative deviations from the main plot that contribute to the resolution of the story. For example, in "Star Wars: Episode VI - Return of the Jedi" (1983), Han Solo's preoccupation with Leia and Luke's relationship is a sub-plot resolved by his discovery they are brother and sister.
Write the first draft of the script, using a computer and an approved software program for proper formatting. In the first act, introduce the main characters and establish their goals. In the second act, confront the hero with a series of setbacks that reach a climax. In the third act, show the characters dealing with the consequences of their success or failure. Each act ends with a turning point, or event, that leads the characters into the next stage of the story.
Prepare for re-writes. Most first drafts are heavy on exposition and dialogue; cut the excess story details from your work. Look at your three-act outline as a reference and track deviations from the original road map. Ask if they are valuable sub-plots or distracting detours from the main story. Avoid giving screen direction (zoom in, zoom out). Focus on character and how character traits are expressed through action and dialogue.
Pass the script on to friends or fellow screenwriters. Although you might think your script is perfect, listen to their suggestions for improvement carefully. Re-write the script as many times as it takes to make the characters believable and their struggles interesting. This may be a long process, but if the script is ever to be made into a film, it is a necessary one.