This page explains how to start and complete the The Korok Trials side quest.
How to Start the Quest
Follow the instructions in The Hero's Sword to reach the Lost Woods. When you get there, talk to Chio, the large Korok with a yellow leaf on his face near the hollow in the Deku Tree, to start the quest.
How to Complete the Quest
Chio will give you three Big Hearty Truffles after you complete all three trials.