I had to take a speech class in college. The demo speech was fun I'm sure your teacher has already said this, but "Show what you know". That the simplest way to do great, have fun, and give the class something to ask questions about.

I chose "How to give an insulin injection" because my daughter has Type 1 Diabetes, she was able to attend the class and I simply dosed her in class, explaining the steps. I gave background prior to the demonstration about why we use insulin and how a blood sugar reading is used in a mathmatecal formula to find out how much insulin is required and when, and then briefly discussed the expected outcome of the insulin dose, and then took Q&A.