Instructions
Travel to Jubilife City, located north of Sandgem Town, and enter the Global Trade Station located next to the Trainer's School.
Speak to the receptionist behind the desk at the back of the GTS.
Select "Trade," followed by "Yes" to each of the three questions to save your game and connect to the Nintendo Wi-Fi Connection.
Select "Seek Pokemon" followed by "Pokemon."
Select "ABC," followed by "Celebi."
Select "Search" to begin searching for other trainers who are offering a Celebi. Tap any of the seven trainers that appear to view their information, such as the statistics of their Celebi and the Pokemon they want for it.
Press "A," followed by "Yes" to trade your Pokemon for the Celebi. Your Pokemon is removed and given to the trainer and you receive that trainer's Celebi in return.
Press "B," followed by "Yes" to exit the trade menu once the trade is complete. Select "Exit" to disconnect from the Nintendo Wi-Fi Connection and re-enter the GTS building with your Celebi.