Are there any instructions of a possible export-import to actual Godot projects from Material Maker?
There is an old release of Material Maker in the Godot AssetLib. If you want to test it, just install it, activate the extension, and click the "Material Maker" button in the top right corner of Godot. In this integrated version, File->Export will generate a SpatialMaterial .tres file that you can use directly.
The new one (0.6) is not integrated yet in Godot. There's progress in the export_plugin Github branch, but I'm not satisfied with it yet. It will be available in the AssetLib whenever it's ready.
If you use 0.6, exporting a material generates a few png files. Then you will have to create a SpatialMaterial in Godot, use ..._albedo.png for albedo, ..._orm.png for ambient occlusion (R channel), roughness (G) and metallic (B), ...depth_texture.png for depth, ..._normal_texture.png for normal map and ..._emission.png for emission.