This is actually much simpler than one would think and your checklist is as follows.
- Personal Domain name
- Server side application
- Client side application
With #1 you have some options. You can purchase a domain register for a free one or use your public IP as is.
Through port forwarding on your router you broadcast your public IP address to the world and allow other computers to connect to it
The server application handles how your game would run and the client is basically just a player side observation of this.
While simple in practice some knowledge is required to achieve the desired results so research the following.
Godot Multiplayer Framework
First test your Multiplayer game on localhost then try it on a LAN and once it's all good you can host the server yourself(that PC always needs to be on) or run a Dedicated hosted server through a company.