Develop an idea. You may have the basis for a story already in mind; write notes to yourself, or construct an outline. Scribble character names and backgrounds, and design the world your characters will inhabit. If you find yourself looking for ideas, examine books about mythology or history, or read the newspaper to locate interesting stories.
Determine who you are writing for. Is this a humorous movie aimed at children, a romantic period piece or a gritty science fiction thriller? Your target audience will affect what sort of dialogue and visuals you can use, as well as the content of the storyline.
Write your first draft. You can write in linear fashion, from start to finish, or construct specific scenes as they come to you. Jump around if you like, experimenting with dialogue and imagining what sort of camera angles would work best. This process can take anywhere from days to months, depending on how long you're willing to work on your script each day.
Leave your completed draft for at least a week. Return to it with fresh eyes and examine it for inconsistencies, plot holes and awkward language. Speak your dialogue out loud; if it sounds awkward to your ears, it may sound awkward onscreen, too.
Each scene has a heading, which describes the location your characters are in. Headings are always capitalized. For example, "INT/SPACESHIP/NIGHT" means your scene takes place inside the spaceship while at night. You may add additional information as you need to, such as the year, or whether this is a dream sequence.
Describe your scene. Underneath each heading, write a brief description of what's going on. For example, "ECKHART walks through the dimmed corridor. Water drips; he turns around to look. When he sees nothing, he smiles nervously." This description is generally sparse, and serves to give filmmakers and actors a template to base shots, set design and character reaction around.
Capitalize all character names. Underneath each character name is what David Trottier calls "actor's instructions," where the writer can suggest facial expressions, tone of voice and other mannerisms to go with the dialogue. For example,
ECKHART
(half-smiling, his scar visible)
If I knew why, I'd be writing.
(looking at URSULA)
I see you smirking over there.