These are the steps I took

  1.  Downloaded the assets from the asset store (Terrain Tools,  Free Speedtrees, Grass Flowers Pack Free).
  2.  Went into the Package Manager tab and went into advanced settings. Clicked on Show Preview Packages and saved Terrain Tools.
  3.  Right Clicked Hierarchy and then 3D Object and then Terrain
  4.  In my Inspector tab, I clicked the paint brush and pressed on Sculpt/Bridge. I place rocks around.
  5. Clicked Paint Texture and then Layers to add rocks, moss, sand, etc.
  6. Painted across the terrain
  7. Clicked on the tree symbol in the Terrain tab and hit edit trees, after edit trees, I clicked on which trees I wanted to be added.
  8. Clicked on the flower symbol, and clicked edit details and added grass.
  9. Went into my lighting tab and added fog