I’m on windows and have gone through loads of options and googling to see how to change a notepad .txt to a .rot and haven’t found anything.
Unless I completely misunderstood what you’re after: If this only concerns the file suffix it is probably the easiest in Windows Explorer, with the file highlighted to hit F2 (Rename) and instead of a .txt suffix, just use *.rot in there. Windows will likely complain but as you’re just changing the file suffix this doesn’t matter.
Otherwise the instructions above should explain how to set up a rotation file from scratch (both on Windows or Mac). As Windows’ built-in Notepad app sucks, I suggest to use something like Notepad++ or VSCode (see link above).
Christian