Location: Undvik (Skellige), then Kaer Trolde Keep (Ard Skellig) or Gedyneith (during Secondary Quest: Coronation).
Speak to Hjalmar in Kaer Trolde or at the Coronation. If he rules Skellige, he has sworn fealty to your cause, and will arrive at Kaer Morhen without question. If he isn’t King, Cerys agrees to send him as her representative—either way, he’s coming! To ensure he becomes ruler, you must have completed the following:
Secondary Quest: The Lord of Undvik.
Then 
Then