With this 3DS subscription service, you also get a free legendary monster for your collection.
That's right: install the Pokemon Bank app before September 30th (and follow these instructions) and you'll get to enlist Celebi in your Pokemon X and Y party.
A quick note: if you're reading this tutorial AFTER March 14th, 2014, the free trial program will have ended. So, you'll need to pay an annual £4.49 / €4.99 subscription fee to get access to Pokemon Bank. Also: Hi! How is the future?
First, download Pokemon Bank from the eShop.
This bit's easy. Make sure you're connected to the internet and then boot up the Nintendo 3DS eShop. Next, find Pokemon Bank. If it's no longer on the home page, you can use the search bar to find it.
The app is free, so just hit 'download' and wait for it to transfer to your device. When it's done, quit the eShop, unwrap your little digital present, and prepare to load Pokemon Bank.
Make sure you have Pokemon X or Pokemon Y in the cartridge slot. You don't need to do this if you downloaded the game from the eShop, of course.
You'll also need to have a save game where you have progressed far enough to unlock the Pokedex. That's pretty early in the game, but you'll need to go through the basic tutorial in the game before Pokemon Bank will work properly.
Now, open Pokemon Bank. You may have to sign in to, or sign up for, a Nintendo Network ID if you haven't already done so.
After a few introductory setup screens, you'll come to a page which reads: "Your pass has expired." Don't worry - just choose 'purchase' and you'll be taken to a screen where you can "buy" the trial pass for free.
Now you have 30 days to use the service before you have to pay.
Choose 'Use Pokemon Bank' from the menu. You now get to pick the Pokemon game you want to use. So, choose your main game.
You'll also get some free Poke Mile or Battle Points if this is your first time using the software.
You won't get Celebi unless you actually use the Bank, so let's put a critter in the cloud.
Place your cursor over a Pokemon on the bottom screen, press 'A', and choose 'Move'. Then, move up to the top screen and press 'A' to drop the 'mon into the Bank.
Hit 'X' to save and quit. Back out of Pokemon Bank and quit the app.
Load up Pokemon X or Pokemon Y and choose 'Pokemon Link' from the main menu. This enables you to collect the gift from earlier (the Miles or Battle Points), and frees up your Link for Celebi.
Open Pokemon Bank again. Now when you go to use the Bank, your guide will give you a free Celebi as a gift.
Hooray! Save and quit the Bank app again.
Finally, load Pokemon X or Pokemon Y and choose 'Pokemon Link' again. Choose 'receive gift' to get your Celebi.
Your new legendary monster will be stored in a box, so just find any PC (there's one in every Pokemon Center), withdraw Celebi, and add it to your party.