Brothers In Arms: Skellige
This quest begins automatically upon completing The Ugly Baby Story Quest. For this quest you'll need to travel to Skellige and speak with three different people to ask them if they'll join your fight at Kaer Morhen. If this is the first Brothers In Arms quest you're doing then let me tell you how it works; in order to ask the person for help you will typically need to complete their quest chain.
Two of the people you can ask to join you in Skellige are tied to the same quest chain which makes things a lot easier. The other person who joins you, Ermion, doesn't require you to complete any quests. He joins you with no difficulty. Here's a list of everyone you can get to join you in Skellige and how to get them:
Ermion: Fast travel to Gedyneith and speak with him in the Druid's building
Crach: Speak with him in the Kaer Trolde Castle. He won't join but he'll give you a weapon
Folan and Vigi (Optional): If you met these two and kept them alive during The Lord of Undvik quest then both of them will join you at Kaer Morhen too
If you're having trouble figuring out how to find Cerys inside of Kaer Trolde then I recommend you use my compass below for directions. I'm standing in the main hallway just before the large room where the bears killed everyone during the King's Gambit quest.
To reach Cerys you'll need to go up the stairs at the end of this hallway; don't follow the white dots on your compass in this instance since they lie.