This can be done once you have completed year 6 and transported outside with the Tent for fast travel for the first time.
Go inside the tent and you will see this view in the image below, ensuring that you are Harry & Hermione:
Now activate the stereo with
which is shown in above image which is left of the red beam of light.
Then as soon as the music plays and Harry & Hermione dance you will have that magic pop achievement unlocked.