This one is quite a simple side quest that obviously involves fishing. In fact there isn’t much to this one at all actually.
In order to trigger it though, you are going to have to progress through the story enough until you unlock the ability to travel to Man Yuan Temple.
Once you are able to do that, progress toward that Temple without actually going to the place itself as the area you want is just past the temple.
After arriving and speaking to Mr Huang you will learn of a fishing competition and the winner receives a free shirt. It is quite simple to win this competition as long as you keep fishing. As you can see there is a local fishing area for you to fish at anyway, which of course helps.
For this I had managed to gain enough money in order to rent the best fishing rod available, thus you may get different results if you cannot afford it, so perhaps grind a bit in order to be able to afford the best rod that Mr Huang has before completing this one. It is up to you.
Anyway after renting out a fishing rod head to the nearest fishing spot and fish. All you need to do in order to catch a fish is rotate the R3 stick (PS4). The game actually shows you.
Keep fishing until 3pm in-game and you should by that time of caught the biggest fish in the competition, report back to Mr Huang to end the quest.
Related Guides / Links: