Travel to the Room of Requirement by fast travelling through the secret areas section of the Hogwarts Map. Weasley will be waiting inside the room, talk to her to begin the quest.
Gather Moonstone
In order to proceed, you will need to have 15 Moonstone in your inventory. You should have plenty from casting Evanesco on the piles of chairs from the Room of Requirement quest. If not, you can cast Evanesco on anything built inside your room to get the Moonstone refunded.
It’s possible you never actually see this objective, and that’s okay! That means you already have the Moonstone you need.
Return to Professor Weasley
Simply talk to Professor Weasley again to continue the quest. She explains that the Conjuring Spell can be used for more than actually productive tasks, you can also decorate the area as well!
Conjure Wall Decorations x5
To fulfill this objective, you will have to use the Conjuring Spell place five different decorations that can be placed on walls. This includes mirrors and paintings, and the items do not have to be unique.
Conjure Floor Decorations x5
Similar to the prior objective, you have to place five decorations on the floor. They can be any rugs, statues, lamps, or tables available to build.
Speak to Professor Weasley
After your decorating spree, talk to Professor Weasley one more time. She will teach you the Altering Spell, which you’ll be using for the next few tasks.
Alter the style of an item
Aim at any of the decoration items you placed, and cast the Altering Spell. Select a different style using / and exit the menu to fulfill the task. You can do this with any item that has multiple styles.
The Altering Spell in action
Change the color of an item
Similarly, simply aim at a decoration item with multiple colors, cast the Altering Spell, and change the color of the item you selected with / .
Adjust the size of an item
For one last time, cast the Altering Spell at an item with multiple size options like a mirror and change the size of it using / .
Alter the balcony or floor of a room
Finally, some variety in the quest! Now instead of casting the spell at an item, you’ll be casting it at either the balcony or the floor. Select any change you want for what you select and you will automatically talk to Weasley.
Speak to Deek
To wrap up the customization of your room, speak to Deek near the entrance. Deek has the ability to change the atmosphere of your room to fit the aesthetic you’re aiming for. This will be a complete overhaul of the room’s aesthetic and you can change it as many times as you like, so don’t feel pressured to make the perfect choice right away.
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/ 
and exit the menu to fulfill the task. You can do this with any item that has multiple styles.
