One of the best ways to learn to be a good supervisor is to sit back and think about our past supervisors. If you are just out of High School think of your teachers or summer job bosses. Over the past forty years I have worked for some great supervisors, a bunch of good supervisors and some poor supervisors. From each of them I have taken away a piece of their style and made it mine.
From the poor supervisor I took away a lot of what not to do. From the great supervisors, I took away as much as possible. You need to remember that we can not become a carbon copy of a great supervisor, but we can learn to use some of the parts that fit our own style. From the good supervisors there is always a bit here and there that we can take and make ours.
In addition to the above, ongoing professional readings and taking some classes or courses on supervision related topics is a life long process.
The bottom line is we learn from each experience we have to include professional reading.