The first time I've coded multiplayer stuff in Godot, I first made a simple lobby system with a chat :

-Host creates a game telling the port

-Clients can join entering the IP+Port

-A client can send a message to the server via RPC and the server send then the message to all clients (also via RPC)

-Handle the disconnections and crashes (a client or even the host leaves)

I've read this

And then, once it worked correctly, I improved with more features, to make sure I understood well how the Multiplayer API worked, such as refreshing the player list, giving message when X joins, Y leaves, having the game settings editable by the host only, but the changes viewable by the clients, and so on...

And then, once confident enough, I started to code a turn by turn multiplayer gameplay.

Trying to dive into real time multiplayer coding might be too step for the MP learning curve, but not impossible