To unlock the Home Again trophy in Kingdom Hearts III, you will need to have to have finished Monstropolis in order for Hundred Acre Wood to become available. Once you have finished Monstropolis, you can visit Merlin who is located in the Le Grand Bistro in Twilight Town near the spawn point.
When you arrive at the Bistro, you'll be shown a book that you can use to teleport yourself to Hundred Acre Wood to meet Winnie the Pooh and his friends.
At Hundred Acre Wood, you'll be asked to complete three mini-games. Once they have been completed and the cutscenes have ended, you will have finished the world and unlock the Home Again trophy.