For this achievement you need to play guitar during a camping event.
To obtain the guitar you need to travel to the VIP private beach inside Chogeko, then jump from the ledge of the tallest rock in the area. As soon as you jump, some dialogue will pop, then Yu will say that she found a secret cave entrance. Once you interact (Dive) to the underwater cave you will find a Souvenir Conch.
Next time you come home, you can activate an event where Kay gives the guitar to Yu (I think you need to eat/sleep first).
After that you need to cook and eat in a camping to activate the event that she plays guitar. Personally I did it in Nekako, eating the double banana preparation in the cooking menu.