Instructions
Travel to the Pokemon Center in any city. Go downstairs to the Wi-Fi level.
Talk to the central attendant and select "Yes" when she asks if you would like to enter the Wi-Fi club. Save your game and enter the room.
Go up to any person in the room and press "A." Select "Trade" to begin trading.
Select your Pokemon and wait for the other person to select their Pokemon. If you are fine with the trade, select "Confirm." If not, select "Cancel." Once you press "Confirm," the trade will occur.