My friend wants to know how...
First you must get a Nintendo ID if you want to know how click here. When you got a Nintendo ID you can go to E-Shop and download it. When you are about to acces it you must have the Game Card inserted. When you logged in and everything they will ask you if you want to download Poke Transporter and you can buy it. You also get 100Poke miles or 10BP. When you have used it the first time you can get the Celebi with Hold Back.
Hope did something good.
You go to the Nintendo E-Shop, search for Pokebank and click download :I Once installed click on the icon and use it.