Depends how complicated you want to get with it, my original inktober renders were unlit, I discuss it here:
https://twitter.com/animtree/status/1290597942496305155?s=19
Now I'm using a simple 'up vector mask' for the snow shader on all the rocks. There's plenty of examples of this available online.
The terrain shader is by Flatkit which gives me pure black shadows, but I still want to build my own solution and improve upon it eventually.
We were discussing the shadows over here https://twitter.com/csims314/status/1299853959386009607?s=19