To unlock the Fly Free trophy in Little Nightmares 2 you'll need to throw a paper airplane through the window in Chapter 2 (the school chapter) You can find the paper plane in the room that is just after the blanket rope you climb to enter the school, the window that you have to throw it through is the one that the rope blanket is attached to. Here is my own video guide that shows how to get the Fly Free trophy. The guide for the Fly Free trophy is at the 7:57 mark in the video.