As you make your way around Hateno Village dishing out mushrooms during 'The Mayoral Election' quest, you'll meet Koyin at Hateno Pasture. Upon speaking to her, she'll inform you she needs a little help, kicking off A Letter to Koyin.
Let's take a look at how to complete this A Letter to Koyin quest guide.
Zelda: Tears Of The Kingdom: How To Complete 'A Letter To Koyin' Side Adventure
Koyin can be found at the lake shore at Hateno Pasture and in order to help her out, we're going to need to retrieve a bottle from the water in front of her.
Luckily, there are plenty of trees around to aid us here, so get to chopping and attach a few logs together using Ultrahand. If you've got a fan you can make this very quick and easy by creating a little motorboat to reach the bottle.
However, even if you're stuck and out of Zonai tech you can make do. We blew up two trees (we were out of swords and axes!) and then used the logs to make a little bridge out to the bottle. Whatever way you choose to get there, just make sure to use Ultrahand to bring the bottle to Koyin and lay it at her feet, as you can't actually pick it up.
Once you've done this, Koyin will excitedly invite you to her house to watch her make Hateno Cheese, eventually making so much that it'll become available in the local shops.
Now that the cheese has been made, you'll be rewarded with some, and you may want to take the recipe and cheese over to Mayor Reede to complete another of the game's side adventures, A New Signature Food.
For more handy guides, check out our Zelda: Tears of the Kingdom walkthrough for all of your adventuring needs.