Hey Guys & Gals!
This is my 100% Achievement/Trophy Guide walkthrough (& All Collectibles) for 11-11 Memories Retold and is divided into 2 parts.
This happens in Part 2 - Chapter 2.
So this takes a little time, but after a cut-scene Harry will say "Look Kurt, I've made a rat trap... I think"
Simply go where i go in the video and it unlocks a little bit later on.
It starts at 1:32:30 showing you where to put it, you will now have to go and do some story related things, and you come back later to pick it up. Part 1 video.