Again, there are no rules; however, there are just a few suggestions.
PowToon suggests a five-step scriptwriting recipe in The Power of Cartooning Marketing e-book. It covers Your Challenge, The Steps (1-5)*, Your Script, The Conversation in the Viewer's Head, and Your Visuals. It provides an example with all the details filled in so that you can see how you can build up the details.
I have a storyboarding template on my website (http://www.animatedcourses.guru/) that you can download, save, and edit if you want to. It contains hints and tips and also has a space for the "elevator pitch".
My template separates out the "script" for voiceover (if any) and the "words" that need to be on the screen, as shown in the following screenshot. ...