Step one: Remove old string
Step two: Put new string in
Step three: Stretch new string
Step Four: tune new string and play for a while
Step Five: Trim new string at the tuning machine
Step Six: Let guitar sit for a while
Step Seven: Tune string again and play away!
If it was me, I would write an outline starting from the above. Under each of those steps, I would write some phrases that I could think of to more deeply describe the step. You didn't say how long the speech is supposed to be, but if it is 10 minutes or less, that is probably all you are going to need. If you are expected to talk for a half hour, you will need more. Some additional ideas would be:
1. How to select a replacement string
2. How to check intonation
3. When to check intonation, example: if your replacement string is a different gauge than the string you were previously using
4. Things that are good to check when the string is out, examples: nut condition, tuning machine adjustment
5. What can go wrong when changing a string
Hope this helps.
Mama Mama many worlds I've come since I first left home.