Well, what's your audience? Identifying your audience is the most important part of creating a training/presentation. Are these people who have never used email before (that would be weird) or people who've used some other client (like Outlook on a PC) or web-based email (like Gmail)? You have to know what your audience is and approximately what their experience level is.
If you know what they are familiar with, the best methodology for the training would probably be compare and contrast, i.e. this is how you set up your mail account in Outlook, and this is how you do it in Mail. Here's how you do this task in Mail that you are already familiar with in XXXX.
I train customers for a living, and I know it can be difficult coming up with a training from scratch. But figure out what your audience is, create an outline of what you need to get across to them, and the slides easily fall from that outline. Intersperse your presentation with demonstrations of Mail, and your training will be a success!
Steve