Very important: It seems to be best to do this after the episode "Magic is Might." The Marauder's Map--the item you need for the achievement--seems to sometimes glitch and disappear once you start the next episode. (If it does, exit to Diagon Alley, and then follow the ghost studs back to the tent, and hopefully it will turn up. Thanks to Flyboy80498 for the feedback and tip).
As soon as the cutscene after "Magic is Might" ends, you'll spawn outside a tent. Go inside, and look to the left. The Marauder's Map is on a table. Simply pick it up, and you'll see Hogwarts through the eyes of Ginny and Neville. The achievement pops immediately. After that, if you don't want to explore (there's nothing new to see), follow the ghost studs to the top of the stairs and click on the "Wanted" poster to go back to the tent.
This is also the place where you can get the "O Children" achievement, by having Harry and Hermione dance in the tent.
Good luck!