Professor Weasley has taken a close interest in your progress at Hogwarts. In this mandatory Class Assignment, you’ll need to find a couple of Field Guide Pages in the castle, and then attend Transfiguration Class.
After completing Fire and Vice, you’ll get an Owl mail from Professor Weasley, asking you to acquire two Field Guide Pages in Hogwarts Castle before attending Transfiguration Class. While you can theoretically do them in either order, the compass will guide you to the Underground Harbour first, so that one’s easier to do. The closest Floo Flame is technically the Viaduct Courtyard flame in The Great hall, but it’s likely that the closest one you have is the one in the Central Hall, Library Annex Wing.
Collect the Field Guide Page in the Underground Harbour
Make sure you’re tracking the quest (you can do so from the Quests menu if you aren’t), and then tap on the d-pad to be guided to the entrance to the Underground Harbour. If you’re starting from the Central Hall, you’ll go up the southeastern stairs, and continue up more stairs and outside. Across a stone bridge and up another set of stairs is the entrance to the harbour — enter the Underground Harbour by holding /.
Once at the harbour, you’ll first want to cast Disillusionment and open the Eye Chest that’s at the end of the left pier (1.1). Then go out onto the right pier and cast Revelio where indicated (1.2) to find the Field Guide Page you’re looking for (2). When it appears, acquire it by pressing /. Casting Revelio will also have revealed a chest behind you at the end of the pier, and a bit more loot nearby to the east-southeast, so grab that now.
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Collect the Field Guide page from the book on Intermediate Transfiguration in the library
Open your map and go to The Library Annex wing, then fast travel to the Library (or the closest Floo Flame you have). Once again, press for guidance, and follow the golden thread as it leads you up the stairs (1) to the second floor of the library. At the top of the stairs, turn right twice to go east — the book you need is in the eastern corner of the second floor. Unfortunately, someone’s already got it! Walk up to Sophronia Franklin and press / to speak with her (2).
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She’ll quiz you on your Wizarding World knowledge, but don’t worry about getting the answers right — you just need to finish the quiz. If you want, you can have her ask you more questions, or you can just tell her “Not now” when the quiz ends. She’ll put the book back on the pedestal, and then you can cast Revelio to make the Field Guide Page appear — collect it with /.
Attend Transfiguration Class during the day
With both pages found, it’s time to go to class. Open your map and select The Astronomy Wing, then fast travel to the Transfiguration Classroom. Walk into the classroom, and then — assuming it’s day — class will begin. Otherwise, you’ll need to wait by standing in the golden circle and pressing / (or opening your map and pressing in on the right stick).
A typical class cutcsene will play, and then you’ll get to learn Transformation via a spell minigame. It’s one of the easiest ones so far, especially for a mid-game spell. After you successfully learn the new spell, walk over to Professor Weasley and press / to speak with her. When the conversation ends, so will the quest!
Completing this Class Assignment will unlock two new Main Story quests:
Unabashed FromSoftware fanboy still learning to take his time with games (and everything else, really). The time he doesn't spend on games is spent on music, books, or occasionally going outside.
on the d-pad to be guided to the entrance to the Underground Harbour. If you’re starting from the Central Hall, you’ll go up the southeastern stairs, and continue up more stairs and outside. Across a stone bridge and up another set of stairs is the entrance to the harbour — enter the Underground Harbour by holding 
/
.
