Instructions
Enter the garden and turn left at the fountain in front of you. Go through the door and search the shelves to find jars with beans. There is a chest with health in the corner if needed.
Go up the stairs. Break the jars you collected on the first floor by dropping them down to the first floor. There is a crawl space up there with a chest that has card number 12. Go back through the crawl space and down the stairs.
Pick up the beans and stand at the bottom of the stairs facing the wall. Light the Lumos and a hidden panel will be revealed. Walk outside.
Follow the trail until you come to a crawl space leading into a cave. There is a chest with a cauldron cake and another crawl space to exit on the opposite side of the cave.
Follow the trail until you encounter your first three gnomes. Use Flipendo to knock them down. Pick one up after knocking it down and carry it behind the large tree on the left of the screen. There is a hole here and you will be able to press the "A" button to throw it down the hole.
Throw all three gnomes down the hole and the bars in front of the Lumos will disappear. Go up to Lumos and a platform will appear. Get on the platform by using the ledge at the far end of the shed to jump across or run down the hill and back up causing Harry to jump and grab the platform.
Look to the left of the shed when on the platform to see a gold chest with a little ledge leading to it. Jump over to the chest to get card number 10. Get back onto the platform.
Jump to the second platform that you can see from the first. Once landing, the first platform will disappear and another will appear. Continue jumping to the next platform until you jump to a ledge that leads to a cave behind the waterfall.
Go to the left of the cave to find a white flower, which is asphodel, your first ingredient. Four gnomes will appear after you pick up the plant. Drop down outside of the waterfall and deal with these gnomes in the same way you did the first three in Step 5. The bars in front of the gold chest will disappear when the last gnome is taken care of.
Open the chest to get card number 17. Follow the trail back to the entrance and exit the garden. Once you talk to Ron, he will tell you that it's time for bed and you will be done with Hagrid's Garden.