Using the Streamer Overlay - SSBDoppler/slippi-hud GitHub Wiki
This is no longer supported, but it would be fairly easy to recreate if you wanted.
https://github.com/SSBDoppler/slippi-hud/blob/master/tut/streamer.png
Setup your player data similar to how mine is setup. Left player being index 0 and whatever you want to call yourself. Right player to index 1 and your connect code.
Index 0 has been replaced with Port Lower
Index 1 has been replaced with Port Higher
Once connected to dolphin, the overlay should automatically detect what port you are and set the overlay accordingly. It will auto hide when not in game. This does not work in 2v2.