How do I get the Slytherin Crest in Dumbledore's Army?

Slytherin Crest: You need dark magic for this. Take your dark magic user
and switch to the yellow spell. In normal gameplay, this is Focus. For
dark magic users, it's the Imperius Curse. Go to Madam Puddifoot's and
stand outside until you see a yellow aura around the girl inside. Press
Circle to control her and get the crest.

I found this section confusing, but what finally worked for me was to a character with the yellow 'focus' spell (I used Dumbledore), and when I stood close to the window of Madam Puddifoot's and hit 'Y', it switched me to the character inside the shop (just called 'girl' at the top left of the screen where the character's name shows). I could then control 'girl', and walk her over to the crest piece to collect it.

The controls for this were confusing because when I was in range of 'girl', the 'B' prompt showed up with the yellow aura, but holding 'B' did nothing - what you need to do is tap (but not hold) 'Y' to simply switch characters. The 'B' prompt is misleading as far as I can tell.

I did this in the Xbox version, and in free play, so I'm not sure if it would work in story mode, or on another system. I posted this answer because I found that the other answers online didn't explain well enough to answer my question.