Right after exiting the Statue of the Goddess after placing in the Ruby Tablet, you’ll notice Wryna shouting for her lost daughter, Kukiel. Speak with her and agree to help. Go into a house now and sleep until nighttime.
Next, go to the southeastern portion of Skyloft. It is the graveyard I’m trying to direct you to, and is the southernmost point of the east side of the stream. Here, find the large tree nearest the wooden wall. Hit the tombstone nearest that tree with a roll or your sword, opening up the wooden wall.
Go into the alcove and descend the ladder. Go carefully along the path and, at the end, you’ll hear screaming. Go inside to meet with Batreaux, who is just messing with Kukiel. Speak with Kukiel to learn she’ll go home come morning.
Batreaux will then recruit you for the Gratitude Crystal quest and all. Anyhow, go to another bed and sleep to morning. Go to Kukiel’s house (east of the Bazaar, near the river) and, inside, speak with Wryna for your reward.