Advice from a genius
Bobby McFerrin on vocal improvisation: The first thing you must learn how to do is to open your mouth & sing and keep going. That's it. And anybody can do it. Kids do it. The most courageous thing you can do is to set a goal of singing for 5 minutes, you start and you keep going. If you're not used to doing this, you're going to want to stop! Every molecule of your being will tell you to stop for several reasons- it doesn't sound good, you have no idea what you're doing, your scales are wrong, your intonation is lousy, etc, etc...but you mustn't stop. You keep going.