Ineed to know what room and day to awnser your question.

you use glacius on the fairies then hit them with flipendo and the fairie will fall then collect the wings(you HAVE to be hermione) then you have to face the demetors as harry once youve done that youll be back in hogwarts

You will start at the classroom where you learned Patronus Charm, be Hermione. Crawl under the gate and get a new spell called the Draconifor spell. This spell is use to make dragon statues breathe fire, but only in close distances. When you have the spell go to the library and go to the secret path and try to go to the third floor. Try not to be seen. You need to go to the statue of the humback witch that leads to Hogsmeade. You will be at the Shrieking Shack and thats where the faeries are. Then you will meet Draco Malfoy and his gang. Defeat them and you will get to the faeries. Use the the Reparo spell to repair the dragon statue and use the Draconifor spell. When the faeries got out of the bush, use the Glaclies spell to put out the smoke from the wings and use Flipendo to get catch them from the floor.

If u are talking about where u get the faries foe Snape's potion class u just have to catch three of them and then dementors will appear Ron and Hermione will brake down the fence while u keep casting that u learn from Professor Lupin.(i don't remember what is called)