Write the script for the commercial. This needs to include the locations that will be featured in each shot, the actors in each shot and the dialogue or text that will appear.
Create a blank storyboard. You can do this by simply drawing frames on paper by hand, or by using storyboard software if you choose to make a storyboard on the computer instead of drawing it out.
Decide the most important elements of the story. These key points are the first frames you will storyboard.
Sketch a rough image for what you want each of the key story elements to look like. Include important elements such as setting and the people in the shot. The amount of detail is up to you, and depends on your ability as an artist. Focus on creating an image that somewhat represents the way you hope to shoot that particular shot.
Add in additional frames. This is easier to do on a computer program. This could include more specific shots in between the main story points, or things like where to insert the graphics often included in a commercial, such as the pricing or location details of the product or service being advertised.
Add descriptions to the visuals for each frame in the storyboard. Make notes on what elements you need to shoot, what you need to accomplish and any other ideas you have for that particular part of the shoot.