So how do you master the art of impromptu speaking? For starters, just tell yourself to forget about the concept of impromptu speaking.
Instead, you have got to really look at it from a different perspective and that is that you are comfortable, confident, and relaxed, like your best friend is sitting right next to you and you are having a beer or a glass of wine or your beverage of choice and chit chatting with them. Whether you are doing that or standing talking to 10000 people, you are going to feel the same. You are going to think and talk in real time and that is all there really is to do with impromptu speaking.
What gets people keyed up is the thought process that they are out of their comfort zone because they are only used to speaking to a smaller number of people. They get rustled at the prospect of people looking at them and so they start to think about the bullet points in their brain or on their slides. The key is to just act as though you are answering one person’s question even if there is no one asking you any. You can ask yourself the question to start your speech in a conversational manner. That is the ultimate way of getting better at this.
Giving a toast at a wedding or asking a question at a public forum are all impromptu presentations. Try to search for opportunities where you have to think and talk on the spot. For example, if you see a reporter somewhere go up to them and make small talk. Chances are they will want to interview you. Such impromptu speaking can make on the spot speaking very natural and easy for you.