Installing the Client
Download and install Adobe AIR.
Download and install GGPO. Step 3:
Open UDP ports 6000-6009 (inbound/outbound) and TCP port 7000 (outbound) in your home network. GGPO uses these ports to communicate with the matchmaking server and with other players on the internet.
If you need help configuring your router, you may find this guide on Port Foward helpful.
Installing the Game ROMs
In order to play any of the supported games on GGPO, you will need to legally obtain the ROMs for the game you wish to play and put them in the "ROMs" folder in your install directory. You may need to run ggpofba.exe to create the ROMs folder if it's not there after you've installed.