If you want to link a sign to a waystone you need to either discover them or craft and place them yourself. They can be used to designate any destination you’d like to fast-travel to. Then create the sign and select whichever waystone you want it to link to!
You can add multiple signs to a single post and use a directional wrench to change which way they point. And there are larger signs that can hold multiple locations in one.
And you can even paint the signs in whatever color of any block available in the game!