Doing it remotely is a big security risk, File and Printer Sharing on Windows is designed to work only for LAN, you better use VPN for what you want. But if you really want to do it for the heck of it, then please check your windows firewall or any solution you use, typically they only allow File and Printer Sharing inside the internal network, set it to any network. After that you need to configure port forwarding for ports 445, 137, 138, 139 in your router to the old PC you need to expose.