If you follow the road east from the Morthal settlement and its Wayshrine, you’ll pass the Hailstone Valley ritual site and notice a small stone gazebo with a lever at its center. Interact with this lever and you’ll reach the east part of Blackreach: Greymoor Caverns. Just south from your landing location, you’ll find the Lightless Hollow Wayshrine. This is your first “checkpoint”.
From the Wayshrine, follow the road south by sticking to the eastern part of the caves. If it’s easier, keep to your left here the whole time. The goal is to reach the end of the longer passage where the waterfall starts. Once you enter this passage near the Wayshrine, you are pretty much set, as it takes you straight toward this spot. The shard is on the rock that divides the waterfall into two streams, close to the edge, so be extra careful.