Note with hint for getting to the center of Sleet island?

I understand (from checking online) that there's a note (or some other kind of hint?) somewhere telling you how to get to the center island of the "Sleet island" location. I've searched everywhere, but must have missed it - does anyone know where the note's located?

Note that I'm not asking how to get to the center of Sleet island (very easy to find via google) or what the note contains. If you wish to include this in the answer, please put it below the location of the note.

Thank you!