Conceive your story's premise. Consider elements such as setting, subject, characters, change and conflict. If you are writing an epic about war, the subject takes center stage. If you are writing about love, the focus will be on the characters. Write a brief outline of the overall plot, including elements of dramatic conflict and change, where the story culminates in decisive action or ends.
Create profiles of your main characters. Write down the details of how they look; their ages, attitudes and histories; how they dress and their personality types. Include their motivations and how they relate to the conflict and changes that take place in the plot. Make rough sketches of each character, even if another artist will draw the book.
Begin writing the script panel by panel, one page at a time. Start by describing the scene in each panel for the artist, including the setting, details important to the action, and which characters appear and what they are doing. Write captions and dialogue for each character, starting with who is talking, followed by a colon, and then the dialogue as follows. Caption: Later that day, or Spider Man: Oh no! It's the Vulture!
Establish the characters and their relationships to the plot and each other early. Write dialogue designed to illuminate their characters and show their personalities and motivations so it will advance the story.
Create scene descriptions that move the characters' actions toward the central conflict or plot changes. Use dialogue and action to show conflicts or changes. Steer events towards the culmination, or end of the conflict and changes. Write captions sparingly, using them only to help set the scene.
Bring the story to a successful conclusion, again using dialogue and action to show the definitive moments. Read through your script and rewrite it as needed. Show the script to other writers for comments and criticism.
Find a comic-book artist to collaborate with who can draw your script as a graphic novel.