Hi, i am sorry to hear that you were having difficulty. I am an active Unity user and never use Godot, how I set up the auto tile system is using the guide from Unity (the red and green arrow shown). I hope Godot is using something similar system like Unity otherwise Youtube tutorial is the only solution for now.
The only way to set up auto tile system I knew at that time was using the red green guide like shown in Youtube tutorial. Or now actually there is another faster easy way, is using Tilesetter (can be found on Steam). in Tilesetter, there is an option to export the created tile design in Godot file format so maybe you won't need to input the terrain rules manually, perhaps that could solve the issue? Also in Tilesetter there is an export option for Unity as well, I have been using that method when creating terrain tiles.