This page explains how to find the Kuhn Sidajj shrine.
How to Find the Shrine
Go to Keo Ruug Shrine to reach the Korok Forest. Go southwest from the Master Sword pedestal and follow the bean lamps. After you go through several hollow logs, talk to Zooki the Korok and say that you want to do the trial. He says to look for iron in the trees to find the correct route.
Use Magnesis to see the metal rocks in some of the trees. After you find the second metal rock, go southwest to see the next metal rock. Then look south to see the next one. The one after that is also to the south. Then turn west to see the next two. Beyond that, you will see a metal shield on top of a platform. There is a sign behind that platform that says to put the shield into the tree's mouth, so do that.
A metal treasure chest will appear in a nearby tree in the water. The chest contains amber. Use Magnesis to put the chest on the raft in the water. Get on the raft and use a Korok Leaf (there are some in the water) to sail the raft toward the tree in the distance that has torches in front of it.
Read the sign between the torches, then put the chest into the tree's mouth. This makes the shrine appear.
Treasure Chest in the Shrine
The chest in the shrine contains a Giant Ancient Core.