First and foremost for any slideshow or presentation is the script. My writing generally starts with a mindmap. I use this in the beginning to record everything I know about the subject without referring to books. It often includes questions. It is something that, initially, I might spend 30 mins or an hour creating. I find that if I write my thoughts down then I don't forget them. This is the mindmap after just a few minutes thought.
I have used a computer based mind mapping software but a hand-written map is just as useful. I spend quite a time with mindmaps checking points of fact and creating the structure of the the story by moving the thoughts around. It makes the job of writing the script much easier.
How long should the script be? From experience a slideshow for adults should be no longer than 15 minutes of learning which is approximately 4000 words of narration. A slideshow can lead to very intense learning. The viewers do not have to visualise things for themselves: your images should do it for them. For each 1000 words you should aim to make only 5 or 6 main points.
With this intensity of input then you need to give the audience time to digest the information. I think of it as if I am reading a text book. I often flip back a page or two to check a couple of facts to make sure that my mind is going in the right direction. So, after no more than 1000 words put up slides to show your main points as a review for your audience. These pauses are not included in the timing of 15 minutes.
The next stage is to write a straight text and then hone it until it is the best I can do. This is done by reading it aloud and any parts I stumble over are changed. It is no good reading it to yourself: your tongue and lips have different mechanics. Don't rush the reading. Imagine you are telling the story to a friend. Get some intonation to your voice.
Now you are going to need your imagination. You are going to create the shooting script. But I will leave that for my next post.