The Stand of the Sentinels Tallneck can be found in an abandoned treetop settlement towards the West of Forbidden West's map. Kept away from the machines roaming the plains, the raised platforms are a safe haven for the most part. However, actually climbing them to reach the Tallneck isn't very easy. Here's a walkthrough on how to reach the Stand of the Sentinels Tallneck in Horizon Forbidden West to override it.
Tallnecks work in the same way in Horizon Forbidden West as they did in Guerilla's first game. The map is shrouded in two levels of fog with overriding tallnecks removing the first layer.
Therefore, you'll want to head out and override the Tallnecks as soon as you can to clear out the map and reveal all the opportunities.
Horizon Forbidden West The Stand of the Sentinels Tallneck Guide
What you need to do to reach the Stand of the Sentinels Tallneck and override it is find a way onto the platforms in the trees and get to a high enough place that you can jump onto the machine.
The best place to climb up is at the northern end of the base, at the lowest end of the camp. You'll spot a fallen tree with a stump next to it, and a ladder that goes up another tree next to it.
Jump up onto that ladder and turn right at the top, once you reach the platform.
In front of you, you'll see another platform on another tree. The idea is that you wait for the Tallneck to come around and you jump across its lower handholds to get to the other platform. Aloy even suggests that herself.
However, you can actually glide across using Aloy's new glider. There's a handhold to the left of the blue sheet on the other platform. Either way to get across that works best for you is fine though.
Once you're on that other platform, turn right and keep heading higher, making sure you're dropping the ladders on any platforms that have them. Getting higher requires some intricate platforming, so don't be surprised if you fall off.
As you climb higher, you'll need to kill some of the monkey machines, so aim for their faces or tails to take them out quickly.
Once you're at the tallest place in the settlement, you'll see that you can jump to the Stand of the Sentinels Tallneck, so wait for it to come around. You can then climb it to override it and reveal everything nearby.