(I preface all practical posts with the disclaimer that I am not a watchmaker, just an enthusiastic amateur living in a remote part of the world where it is better to learn to do things myself than trust what passes for a watchmaker here in Bangkok)
I live far away from any competent watchmakers. So I have to make do, or wait. I have learned that if I am very patient, and cautious with the correct tools, I can dismantle a watch this far:

So to get the back off I need a tool. Here is my choice:

My default tool up to now has been the Chinese made three pronged unit I bought off ebay for $8. It will remove most casebacks, but I was always wary of coming up against a stiff one, so I acquired the red Horotec unit with various fittings on a visit to downtown LA.
The Horotec unit has interchangeable heads and I use this two pronged fitting that I am not totally at ease with, I would prefer a custom one – but strangely none seems available.
Then along came this:

CNC machined, this tool fits the back perfectly and with flat edged teeth, in a soft metal, that aligns perfectly. This is a prototype but I think it is going to be my go-to opener. I still like to hold the watch in a clamp.
I made a video sometime ago showing how I open a watch, and where to check for the serial and calibre. I could be updated but it is a simple exercise and worth showing for those who, like me before I got into it all, know nothing. We all start from a place of not knowing.