This mystery can be found in the northwestern most part of Jotunheim, on the boundaries of Utgard. You’ll need to climb the rocks to reach the cairn at the top of the cliff edge. Once you do, interact with the stones to begin the mystery.
As with other cairn mysteries, all you need to do to complete this one is stack the stones so that they reach the required height threshold. Although you can do this in different ways, the easiest method involves stacking the largest stone vertically first, then stacking the three other more flat stones on top of each other, before finally placing the longer stone vertically on top, as shown below.
Given the relatively tricky nature of this cairn stacking, you may find that you’ll spend a bit more time on it than others. Stick to it and you’ll soon find an appropriate position to reach the threshold.