https://www.amazon.ca/Back-Stage-Guide-Management-3rd/dp/0823098028
You've got to take blocking/light/sound cue notes, call the show, hold talent accountable for call times, lots of paperwork... generally be in charge of the show when the director isn't there. As a disclaimer, I've never been a professional stage manager because it's way too much work, but I work in professional theater.
If you’ve been a production manager and assistant director, stage management would be a good fit for you. There are many SMs in theatre that are also PMs. Community theatre is always an excellent way to get into theatre!
I fell into stage management in college and I haven’t looked back. Starting as a production assistant for assistant stage manager for a show is an excellent way to learn the ropes. It’s all about trial and error, and the more you do it, the more you will make all the duties your own. You will learn very quickly what works for you and what doesn’t.
There is an absolutely wonderful book called “The Backstage Guide to Stage Management”. It’s very informative and the author throws in funny and realistic experiences. backstage guide to stage management