Get a copy of the brief commercial script from the producer(s) or director. Review with them the key elements that will ultimately be filmed.
Break down the script into the individual scenes. For example, if the script is about a hair care product, it will likely have someone shampooing or getting their hair styled at a salon. A person could be speaking or there will be voice over. Determine the various components of each scene and focus on the key elements, such as the product, sets or location and people involved.
Draw a rough sketch of each scene's key elements. Using the hair care sample, the first storyboard card or piece would show a woman with flat, dirty hair. Scene two shows her walking into a salon. Scene three shows a male hair stylist holding a pretty bottle and showing it to the woman, and so forth.
Complete the sketches until the entire "story" is told in pictures that will later be shot with a cast and crew.
Name the various boards according to the particular moment, scene or dialogue component. So, scene 1's card would have, "Woman realizes hair needs help."
Share your drawings with the director and producers as necessary, to get their approval as well as to verify you've drawn the proper moments. Make revisions as needed.