This will be your first main quest after reaching Skellige. First, as Yennefer tells you, go to the inn and put on the clothes she got for you (it's not a mandatory part of the quest). Don't forget to put the clothes on after picking them up.
Then go to the castle (M13,15). You will attend a wake and some attractions await you there. Each of them will be awarded will small amount of experience. First you can participate in a race to the hill, it won't matter who will win. While raising a glass, you can refuse and gain approval from the sorceress. A while later follow her to the door, there you can protect her dignity by brawling with one of the old men (after what you will be once again encouraged to drink alcohol).
Follow Yen to the closed door, and then walk through the ledge behind the window to the neighboring corridor. Walk to the door on the left side of the room. After interacting with them you will face some animated stuffed animals - wolves and bears. After a spectacular battle, you will find yourself in Mousesack laboratory.
You can look around the laboratory, but in order to continue the search take the vessel from the locker near the bed and place in the hand of the statue standing near the map - by doing so you will open a hidden passage. Behind it you will find the Uroboros Mask that the sorceress is looking for. Immediately after entering the room Earth Elemental will attack you. With the help of the sorceress you will beat it easily.
After the battle, gas will start filling the room. After a moment of waiting, Yen will teleport you out of the grotto. Pick the option "I want to kiss you" to unlock access to a love scene. You must pick another dialogue option that clearly indicates that you want to get close to her. After the whole event you will both return to the wake (or immediately if you choose to skip the scene). After a conversation with Crach an Craite you will finish the quest.