Manorhouse studios, who do some of the most gorgeous terrain EVER are posting up some tutorials about building a diorama. The same information could be used for terrain making as well I guess.
Right now only the intro is available, but more will be added in the days to come.
Dioramas and Scenery
Dioramas and Scenery
Painted Minis in 2014: 510, in 2015: 300, in 2016 :369, in 2019: 417, in 2020: 450