If you look at the map, south of Russafeld there is a set of three small ponds, near the north part of the mountain. You’ll have to climb on top of this tall plateau. Once you’ve succeeded, you’ll spot the tall waterfall powering these small ponds. The very first one hides the skyshard. Although this is a pretty well hidden place, as you get close to it, you can’t really miss the light from the shard that’s lighting up the area. There is a ruined house nearby, but the best marker you can use to guide you is the waterfall. Just keep an eye on it, and you’ll find that 1/3 of the skill point right there.
Russafeld is a farm in central Summerset, close to the Russafeld Heights Wayshrine, west from the town of Shimmerene. It’s easy to miss, as this is the only place of this kind on the entire map, and due to its position, you are most likely to pass it by.