Tabletop Titans has a couple of tutorials on making and basing 1/285 scale trees. I’m going to use these for my 6mm English Civil War project.
Modeling
Making Terrain From Cardboard
Although this video is focused on Warhammer 40K, I think the techniques shown for building terrain entirely out of cardboard are worth studying.
Making Tabletop Rivers
The LudStuff blog has a nice essay with photos on making tabletop rivers.
Poker Chip Terrain Bases
Carmen’s Fun Painty Time blog has a couple nice examples of terrain bases made from poker chips. He’s a very talented guy.