Instructions
Select "Gamecube" page from the Dolphin configuration page. Select "GBA" on the drop-down menu next to "Port 1." You enable additional ports in this fashion if you intend to connect more than one instance of VBA-M.
Open your Gamecube game in Dolphin.
Download and install VBA-M from its website on all computers that need to connect to Dolphin. You can run VBA-M on the same computer as Dolphin or another on the same network. You can also connect multiple instances of VBA-M simultaneously to mimic the multiplayer component of Gamecube games that connect with more than one Game Boy Advance.
Download a copy of the Game Boy Advance bios to your VBA-M directory.
Choose the "Emulator" selection on the "Options" menu. Uncheck the option "Pause When Inactive."
Click the "Options" menu, hover over the "Link" option and select "Joybus Options." Check the "Enable Joybus Connection" and enter the IP address of the computer running Dolphin. If you are running VBA-M and Dolphin on the same computer, use the IP address "127.0.0.1." Repeat this step for every instance of VBA-M that you want to connect.
Choose "Open GBA" from the "File" menu. Select the GBA bios you downloaded previously. The game will start and make a connection with Dolphin.