To get the popcorn achievement in Little Nightmares 2 you'll need to throw the corn that can be found in the hospital chapter (chapter 3) of the game intro the incinerator, this is the same incinerator that you throw the teddy into to get the key. The corn can be found in the room that you have to use the fuse that you get after defeating the hand for the first time. Here is my own video guide for this achievement in case you need help with it. The popcorn achievement is explained at around the 14:42 mark during the video. This video also explains all of the other achievements Little Nightmares 2 has to offer.