In “the olden days” a storyboard was made on 4×6 note cards, drawn in pencil, and arranged freely. This is still a valid method of doing things, although I am getting the impression that with each new generation, these people physically cannot pick up a pencil and write on paper. That “art” apparently has gone the way of the dinosaur. So I will suggest some storyboarding software for the modern generation. But before I do, please take the time to read how we do this with paper and pencil. It is important, and you will get the essentials here, before you go to software.
First thing, you are “brainstorming” here, taking what you have, getting ideas, trying to get some kind of direction (end goal, objective) in sight, and also getting some flow (select elements and put them in order).
The absolutely essential element in your brainstorming is that the means of doing this must be flexible where you can take two “slides” and insert a new one or ten in at that point, and also take a single slide or slides and move them to another place. You can do this with word processing software, or even with presentation software like PowerPoint which is probably the most well-know and serious presentation software.
Secondly, realize that what is on the paper is not the key. It is a representation of a reality in your head. It is a placeholder at this point. So arrange your presentation, and write down things as much or as little as needed depending on how ideas are flowing. You will go back and review what each slide has, and how to get images or design a slide to present that idea.
Again, flexibility is the key. You want your ideas “on paper” so that once there you can analyze them, change them, amplify them, reduce or delete them, or rearrange them. All of this is the brainstorming and storyboarding part of making a presentation. I don’t like the idea of using a big powerful program like Powerpoint for this. Microsoft has a program called Visio to do this before you start in PowerPoint, so PowerPoint really isn’t designed for this either.