Identify your central message. Knowing the main point you wish to make will help you write an effective script and set an appropriate tone.
Determine your limitations. For example, identify any time constraints for your commercial and set your budget. Understanding your limitations at the beginning of the project will allow you to maximize the resources and time you do have available.
Write your script. Make sure the script conveys your key message and is appropriate for your audience. Avoid prejudiced language and profanity. Stay on topic and use humor if appropriate. The goal is for your audience to relate to your message and to be moved to take action as a result of seeing the commercial.
Rehearse your commercial. Make sure the script flows smoothly and everyone says the lines in an appropriate way. Make any modifications to the script as needed.
Film and edit the commercial. When editing, consider adding music or other effects that will help set the mood and reach the audience. Save the edited commercial in a format compatible with the technology you're using to broadcast your creation.