In student public speaking courses, attendees are often given a topic and only five minutes to prepare their speech. What do you do when you are asked to come up with a speech on the spot?
The real key to acing spontaneous speeches is to just do it more and more frequently. The more you get used to it, the more you become comfortable doing it. Imagine that you encounter one of your best friends one morning and they ask you about your weekend. Now, you would not ask them to give you five minds to think and formulate an answer; you just talk. That feels easy because there is no pressure on you and it becomes easy to think and talk at the same time similar to what you do at a table with friends and family. In a public situation, such as a school or board room, you become stressed and start thinking about what you are going to say five minutes from now and how you are going to try and avoid making mistakes.
The key to facing this task is to get to the point where you are comfortable thinking and talking at the same time. It is possible that you might make a mistake but if you look visibly relaxed, comfortable, and confident while being interesting, most of the time no one will even notice the few mistakes you make. They will just notice that you are coming across as calm, confident, and interesting.
So practice as much as you can, use every opportunity you are given to stand up and speak or ask questions on the fly. You must give yourself the permission to think in the moment and not try to craft a perfect, rote memorized speech. Instead, just focus on thinking and talking on the spot. Here is a bonus tip: have a single sheet of paper with you and write three bullet points. That way, you can glance down and revel in the comfort that you have backup in the case that you forget what you wanted to say.