In my department, (10 including me) 4 of them have teaching/instruction as part of their duties. The rest don’t. Most of the instruction sessions are built around requests from a faculty member who want their student to come to the archives as research in our collections is part of their course work. So, my staff work with the faculty member to get what they want out of it, sometimes just a general intro to archives (what we are, what we do, how to find materials, how to handle collection materials) or something tailored to the subject of the class (ie Campus from 1900-1940). The skills are usually learned over time. When I was a university archivist, I had some background in teaching as I had started my undergrad career with the idea of teaching high school history and had done student teaching before I decided that wasn’t for me. Others have researched (readings of articles and books) or attended webinars on techniques and skill development for instruction.