Follow the road from Hakoshae Wayshrine to the west until it changes course. At one point, just north from Orcrest, you’ll notice a small crossroad with a tiny path leading west. North from this exact crossroad, there is a trail that takes you down to the lower parts of Ashen Scar. If you zoom in on the map in this location, you can spot a small divide in the mountain range. Find this trail, and you’ll also find Vasterie, the NPC that gives the POI-related quest on the way down the hole.
This specific trail has a long and large stone overpass. This is the ridgeline you are looking for. No matter if you are coming from the east, from Ashen Scar, or from its central part, you’ll notice this small gap in the mountain that makes the trail visible. To find the skyshard, go up the hill once you find the trail, and go over the stones that form the platform edges. The shard is on top of the overpass. If you pay attention you’ll notice it easily, as there are no trees or other elements that would fog a traveler’s view.