Guest-speaking soon? The best plan may be almost no plan.
Each audience is a unique mix of individuals, and each individual is in a constant state of change. Even if you are speaking about the same topic to similar audiences, each presentation is a unique chemical reaction, something you can't fully anticipate.
Consider ditching the script. Plan to set the initial direction of the conversation and then invite your listeners to co-create a dynamic chemical reaction with you. Ask for input early and often:
You came today hoping to get something out of this. What are you hoping for?
Do any of you have previous experience with ______? What was it like?
I just put a big idea out there. What are your reactions?
I see some curious faces. Am I making sense?
We have more time. What else can I help you with?
I've put a lot of ideas out there today. What stands out in your mind?
Fringe benefit: a lot less time and effort developing your script/slides before your talk.