This image shows Yujin’s location on the map, just look where the red arrow points toward.
Approach her and speak with her, then exhaust the dialogue options by picking the topmost one. You will eventually get to say that you’re helping “that teacher” with an errand.
Yujin will give you a package, it’s just called “Clothes for That Teacher”. Doesn’t leave a lot to the imagination, does it?
Anyway, just fast-travel to the Trove of Verdure Star Anchor in the Cloudford area of The Xianzhou Luofu.
Go to the container in the southern port area, west of the Star Anchor. The image above shows you where to go, just follow the red arrow’s direction.
Interact with the large green container here and deliver the parcel you just got from Yujin. “That Teacher” will open the container and reveal himself.
He’ll take the parcel out of your hands and then disappear, but at least he left behind a chest! Open the square chest he left to get two relics of the Thief’s set. You will also get some Stellar Jades and Strale.
That’s everything there is to know about where to find that teacher’s clothes, and what to do with them.