I am a relatively new tenured professor. My responsibilities increase gradually, and this year I have started supervising students' thesis: a bachelor thesis and a master thesis. (This is in Europe, so master thesis is not PhD-lite).
Somehow it feels like I am expected to already know how to be a supervisor. There are no resources anywhere that I can find. The training I received at the start of my tenure focused exclusively on classroom teaching, and my PhD/postdoc only prepared me to be a researcher, not a supervisor. Of course, I already know more or less what a supervisor does - after all, I have been supervised through many academic endeavors myself and I was able to observe. I also have experienced colleagues who have supervised students for years that I can observe or to whom I can ask advice.
But I don't think that this is "good enough". There are certainly some skills that I can improve, some issues that I can plan ahead for, etc, that I haven't noticed while I was myself supervised: after all, I was busy with my own project at the time, I was not learning how to supervise. I'm also sure that there are some things that were done / should have been done by my former supervisors that I can't have noticed (e.g. how to set an agenda and stick to it for a meeting, how to adapt to roadblocks, etc). Moreover, like everyone, my own experience is unique, and the students I'm going to supervise are not me: what do I do if a student loses a relative, becomes discouraged, or on the contrary outperforms everything I've expected, needs more guidance than I can provide, needs a less hands-on supervision, needs career advice about a career I've never thought about...???
I realize that this is a huge task, so I am certainly not asking for a full answer right now - knowing stackexchange's reputation, such a question would probably get closed immediately for being too wide without concern for whether the question would actually be helpful to myself and, I hope, many other people. Anyway. I will instead ask for resources on supervisory skills: books, websites, classes, etc. What have you found that helped you become a good supervisor?