Free GM Resource: The Terrain Tutor
The Terrain Tutor has about a bazillion videos and not all of them are ones I want to sit through, but there are a lot of great tips, tricks, and how-to videos well worth your time.
I particularly enjoy these playlists:
Back 2 Basics for Wargames Terrain and Scenery Builders
How to make simple wargaming hills
Quick and Simple Tips
There are several more playlists to watch as well.