Break down your script into cels. Cels should move the plot forward in some way, either through movement, dialogue, action or narrative. It even can be as simple as a city skyline to establish a setting or a character's reaction to an action in the previous cel. Draw a line denoting the start and end of each cel in your script, and number them.
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Draw four squares on each sheet of paper along the left side. Leave enough room on the right side for notes about what is happening in the cel, as well as a bit of space between each square. Number each square, making sure there is one for each cel in your script.
Go through your script, cel by cel, and write the details of it next to the corresponding square in your storyboard. These details include location, characters, dialogue, narration and sound effects.
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Sketch out what each scene will look like in the squares. They do not need to be detailed; just enough so you get a sense of what will happen in each cel. Write down the size of the cel directly beneath the picture; sizes range from 1/8 of the frame to full page. Some comic books include unconventional cel sizes and shapes, so always refer back to the script to determine the best way to frame the action for each cel.