For this achievement you have to collect a set amount of crests that have been scattered throughout Hogwarts, most of them are easy to find but I recommend you do this following a guide so that you don't accidentally miss one!
This achievement requires you to collect 48 of the 150 crests, as an additional reward, you'll receive Exploding Cauldrons in prank boxes scattered around Hogwarts.
I've linked in some videos, they show you the location of all 150 crests.
*Credit to DulleNL for the videos