After meeting Hancock at the camp in Thriller Bark, buy the Amazon Lily Eternal Pose there, repair the ship’s Library, and investigate the Sakura Flower during the day. Then return to the doctor’s room in Thriller Bark to obtain the Drum Island Eternal Pose, speak with Kureha at the castle, follow her directions from the forest to the Sakura tree, defeat the Lapahn, and take the flowers back to Amazon Lily; at the coast, choose “Tell them about the flower.”