There are some simple facts of life which we tend to overlook. First, we have a tendency to think of ourselves as being ordinary. But, if you will take a moment you will discover that people you know have a tendency to ask you questions about things. If you examine the questions you will begin to get an idea of what they think are your areas of expertise. On the other hand, you might be thinking that there are lots of people who know more than you do about those topics. For now, we should focus on what we know and what those around us think we know to get an idea of who we are. We are their experts.
Second, when we are asked questions because they think we know, two things are going to happen. Let's say they ask us, "How do I change the depth of field when taking pictures with my camera?" We know we're their "experts" in this area. The second thing that we do when we answer their question is "practice" communication. That rehearsal is valuable because it increases the likelihood that in the future you can effectively explain the process. The more often you are asked and the more often you successfully answer the question the more rehearsals you have had and if asked to explain it to a large group there will be very little problem.
We tend to think of ourselves as something less than an expert and we arrive at that conclusion without proper evaluation. And we tend to think that we haven't rehearsed our communication and therefore are not properly prepared. Rethink these situations because you are very likely the expert your audience thinks you are and you have practiced and rehearsed your communication because you have talked about the subject repeatedly. Now, just stand up and do what you know you can do.