https://mattepaint.com/academy/tutorial/intro-to-terrain-creation-part-1/

Discover the joy of working with procedural terrain creation software, Gaea, in this intro to terrain software tutorial. This is a quick-start tutorial designed to get you up and running with software like Gaea, rather than get bogged down in the details of what all the nodes do and how to control them. By the end of the 20 minutes you'll be able to download the free version of Gaea and start playing around creating your own terrains!