I had some trouble to add grass to godot, the grass mesh was messing up, and not every tutorial tells only what you need to do, but this is very easy.
If you already have a grass-looking mesh and texture, it takes ~50 seconds to add it, using a MultiMeshInstance.
Adding grass: 0:16
Textures & improvements: 1:14
Making a grass mesh in blender (example): 2:34