Instructions
Go to the Cair Paravel throne room and clear out the enemy soldiers.
Pick up the wooden block from behind the throne, and then place it on the pedestal to its right. This will open a secret entrance in front of you.
Pick up the screw from behind the throne, and insert it into one of the two holes on the left side.
Pick up the screw within the secret entrance, and place it into the other hole to the left of the throne.
Open the large chest, which contains Susan's horn.