This tutorial series covers everything that you will likely ever need, to understanding how to create terrains in Houdini. Starting from foundation layer of HF nodes and their functions, to using generated masks to create textures in COPs, to scattering geometry using said-masks, to applying and rendering them in a LOPnet with Karma.

See the rest of the videos and download the project file here:
https://www.sidefx.com/community-main-menu/complete-a-z-terrain-handbook