Please be advised that Netplay over Wi-Fi connection is a bad experience for all players. There is no such thing as a "host-advantage" because all players share the lag and connection. Please try to use an ethernet connection when playing over the internet.
Select Tools > Start Netplay
Set tab to "Connect"
Enter the Host Code provided by the player hosting the game
Configure your gecko codes (explained below) Wait for other players to do the same Wait for the host to start the game
It is not recommended to change the buffer as a connecting player. Let the host decide as they know the correct amount of buffer as mentioned in their part of the guide.
CONFIGURING GECKO CODES
Right click (alt-click) the game in the main Dolphin menu
Select Properties at the top of the drop-down list
Select the Gecko Codes tab
Make sure to use the fullscreen code corresponding to your controller port in netplay.
Since I am the second player, I enable the P2 Fullscreen code.
Now that you have set up your Gecko Codes, you ARE be ready to play.
INFORM your host that you are ready in the netplay chat.