Dolphin's netplay guide to play together in Bigscreen.
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1: Download Dolphin 5.0
https://nl.dolphin-emu.org/download/

2: configure Dolphin
General:
Interface: doesn't matter

Sound:
Gamecube: (very important to avoid desync with gc games)

Wii: (you can enable SD card, doesnt matter, save games won't load over netplay, didn't cause any desync yet)

Path: Here you need to select the FOLDER, not the iso, but the folder for the games to show up in the dolphin list

Advanced:

Next, open the graphics settings:

Enhancements: (you can choose the normal settings but scaled EFB MUST be on)

Hacks: 

Advanced:

How to create a netplay lobby:

Change to traversal server:
Go to host:

Host the game, a lobby will pop up:

Framebuffer has to be set around 20-40 for minimal lag and input delay.

Copy the code for the lobby to the guy you want to play with on steam or whatever.

Press start on the bottom left when the player entered the room and enjoy the game

Tip: when you enabled Side by Side, you will see 2 images display on monitor, I disabled this now for sake of the screenshot but this is normal. Then you need to fullscreen this, and press F6 in Bigscreen (SBS mode)