To transfer an account, please open either the Dokkan Battle Global app or the Dokkan Battle Japan app, depending on which account you bought. Move forward until you can see the starting screen and click DEVICE TRANSFER. 
In the next step, please click on Enter Transfer Code  and continue.
Now you can enter the transfer code that you received via automated email. 
At the end, just click on OK and follow the instructions.
If you are having trouble with entering the code, click on the following link: