Using this plugin require to learn some concept about terrain, This terrain plugin will not work out of the box, so I really suggest to read the wiki which I add recently added, I will add more stuff to wiki but for now I wrote the main concept that you need to know.
Also watching this video will be helpful: https://www.youtube.com/watch?v=PcAkWClET4U
And then this video shows how to use use height brushes to modifying the terrain: https://www.youtube.com/watch?v=e7nplXnemGo
Video about Texture painting: https://www.youtube.com/watch?v=0zEYzKEMWR8
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To downalod the latest release use this link: https://github.com/mohsenph69/Godot-MTerrain-plugin/releases
First clone this repo on your local machine, so you need godot-cpp to exist in GDExtension folder so you can build that, godot-cpp is added as a submodule in this project so to put that inside GDExtension folder only thing you need to do after cloning this repo is runing this code
git submodule update --init --recursive
This will automaticly pull godot-cpp into GDextension folder, After that go inside GDExtension folder and use scons to build this project