Mostly from YouTube and doing lots of experimenting on my own, I particularly like Max Hay's stuff on YouTube, he's recently released a paid-for course which I've done about half of and can say that it's also very good so far. Normally I just try and get a good picture of what I want the outcome to be in my head, block it out in Blender with cubes etc, and then start googling for specific techniques to make things look the way I want.