Devise the story. The basic story should be very clear before beginning to write. Each story should have three acts: a beginning, middle and end. The first act establishes the characters and environment and ends with an igniting incident that alters the character's lives. The second act follows the characters as they try to bring about a resolution to the problem; the second act ends with climax. The third act resolves the problem. This is the basic structure of a screenplay, even a short one.
Write a detailed synopsis. A synopsis will help during the writing of the script. The synopsis is like a skeleton or blueprint for the story. It should include each scene and briefly discuss what happens in it.
Learn the language of screen writing. Slugline or Scene Headings establish where the scene is taking place and are always capitalized. For example, INT. COFFEE SHOP - MORNING. Below the Slugline is the action of the scene. This describes in detail what is happening. For example: "Joan enters, she pauses at the door and looks around. Her eyes fall on Robert who waves to her and she joins him at his table." Dialog is written with the character's name all in CAPS centered in the page. Below the character's name is the dialog exactly as it is supposed to be spoken. Transitions between scenes are also in caps and some common ones are: CUT TO:, FADE TO:, and DISSOLVE TO:.
Write the script. A word processor or preferably a copy of script writing software such as Final Draft will be used. As you write the script, keep in mind all those who will be reading it. The director will be concerned with blocking and the point of the scene. Actors will be interested in their motivation in the scene and background about the character. Other production staff will be concerned with the costumes, lighting and set design. Keep the script to the point and avoid unnecessary language.
Proof the script. Read it out loud; how does it sound when spoken? Does it read naturally? Edit the script for errors.