Tap 設定 (Settings).
Scroll to the bottom and tap データ引き继ぎ.
Tap 発行 and create a password.
The game will generate you an ID and show your password on the next screen, it lasts for one week. Make sure you screenshot this. It's good practice to keep backing up your account every week or two so you never lose any progress.
On your new device, open the game and when you get to the title screen tap the box in the bottom right hand corner. Enter your ID and password then tap the button on the left.
Make a Screenshot after you typed in the Device show you the ID and your PW.