Import the asset into your project or create an empty project.
Go to Demo folders and run them to learn about common use cases.
Examine the code behind the script attached to the Demo game object.
The Demo scenes contain a TerrainGridSystem instance (the prefab) and a Demo gameobject which has a Demo script attached which you can browse to understand how to use some of the properties of the asset from code (C#).
URP/HDRP setup: the asset does not require special configuration in URP or HDRP. However, in URP if you experience rendering problems or if the grid is not visible, try disabling “Depth Priming Mode” in the Universal Renderer asset.