You find Hestu trembling near a grove of trees at Lindor's Brow.
Find Hestu
You will find Hestu at -1696, 1072, 0197, southeast of Lindor's Brow Skyview Tower, which is northwest of New Serenne Stable. Talk to him to start the quest.
Approach the Trees
Approach the trees that Hestu is scared of, and you will discover that they are two Evermean Trees, which will attack you. Fire and axes are especially effective against them.
Give Hestu Korok Seeds
If you have found any Koroks on your adventures, you can talk to Hestu to give him a Korok Seed now, and he will increase the size of one of your inventories. You can choose to increase the size of your weapon stash, or your bow stash, or your shield stash.
If you have another Korok Seed, Hestu will increase the size of another inventory for you, and then he will leave to go east to a place with lots of people. You can find him in Lookout Landing.
Quest Reward
Aside from the increased inventory space, your Character Profiles will be updated with Hestu's profile.