I had an old book laying around from the 80's called "machine language for beginners" by Richard Mansfield that had a chapter (chapter 3, pg 23) on using the system monitor for Apple, Atari, VIC, commodore 64, and PET/CBM. It's focused on primarily the 6502-machine code and BASIC but the monitor section has a step-by-step walkthrough of using the monitor. You even learn how to directly write your machine code into memory and run it, reading, writing and comparing memory segments, etc. I believe the sequel (book 2) is available on archive dot org, but I haven't looked for the first one considering I already have a physical copy of the first.