Draw squares any size, from index card size to full paper size, on one side of poster board. Leave room around the squares for explanatory statements. Use as many poster boards as necessary to explain the full story.
Draw pictures in each square. Keep the drawings simple, using stick figures or rough outlines and minimal scenery. Start with the beginning of the story and draw pictures all the way to the conclusion, telling the entire story.
Explain the scene for each square briefly. Write an overview of the scene, not including dialogue. For example, a drawing of a man walking is accompanied with the explanation, "The man was walking in the park at sunset." Place the explanation on the bottom of the frame unless you know the people viewing the storyboard expect to see it at the top.
Review the storyboard to make sure every scene from the story is there. When the protagonist solves the problem, include this. When the story closes, include this as well. Leave your viewers with a mental image of your story, how you see it played out scene by scene, and how you, as the writer, would explain the action in each scene.