OK, I'm getting started on this. I used the module generator just to create a template module, then I can start adding functionality to it
https://validator.prestashop.com/generator
I'm using Prestashop 8.2.0
I copied the unzipped folder from my basic model to the modules folder of Prestashop (using Docker, if that info is important).
I installed it with module manager. So far so good.
Now, the module doesn't do anything to I wanted to just check if I could get it to trigger with a basic line of code.
The module is set to hook on 'cart creation and update'.
I added one line to the php file of the module
[sorry, the website blocked me posting the line of php code, it's just a simple alert message in the hookActionCartSave function]
and copied the files over to the prestashop server again.
Now, I don't know if I need to force this to update or restart the server to get it to reload the updated module, but when I process a sale in my front end, at no point does the JS alert box from the function appear.
I could be doing everything wrong, just need a little nudge in the right direction.