Read a synopsis of the story you will create a storyboard for. This is essential. To be effective, the storyboard will need to be an accurate guide of what will end up in the final product. In the case of movies, the storyboard often illustrates the essence of how a particular scene should play out, especially in cases where special effects will be used.
Write down the particular scenes you want to illustrate, with a number or some other indication of the order of the scenes. Write a caption for each scene, or a brief text description of what you will draw.
Divide your poster board into squares using a pencil and ruler. The number of squares, as well as the size, will depend entirely upon how many illustrations you want on each poster board. Many storyboard artists use poster board and draw only one illustration per piece. The size of the poster board doesn't matter, but keep in mind you will most likely be presenting the storyboard to directors or special effects people, so avoid using large, hard-to-manage poster board.
Sketch each scene roughly. Some storyboard artists like to add detail, but this is a personal choice. The object is to choose the key scenes or special effects scenes from a movie or story and capture in your sketch what will be happening during that scene. Do the sketch in pencil, then go over it with marker when you are sure it is complete.
Write a caption or short description on each sketch. Also include the reference number. The caption or description you use will likely be something similar, or even the same, as you came up with during your initial outlining process in Step 2.