Using Command-backtick (⌘-`) to toggle between windows
In MacOS 10.13 (High Sierra) I cannot get the Cmd-` to work at all unless the application itself has a menu command to cycle through windows.  If it does, it probably uses that key code; if it doesn't have a key code but has a menu item, it's easy to add the key code in System->Keyboard->Shortcuts.  If it does not have a menu for that, e.g. Safari, then I've found no way to cycle through windows. it does have a Next Tab command.