I always keep my eyes out for good auction and big lots. Won a bunch of trees really cheap, not really knowing their size and anything like that. This is what I turned that bag of cheap trees into.
Bases are cut from thick 3mm cardboard and as always there's a X on the back made with duct tape - this prevents the bases from warping.
I sorted out the trees to have roughly the same amount of big trees for each base and then sorted the smaller ones after that. Fixed the trees in place with a lot of cheap superglue. Made sure the bases have room in the middle, to move around models better. But the 'circle' of trees around it will creat nice line of sight blocking / rough ground.
Lots of pva glue and sand on the bases. Made sure there's plenty of glue around the base of the trees to secure them better in place.
Used cheap spray paints to do the bases. Different browns and greens on the sand. Nevermind some of it hitting the trees.
Add some garden debris to the bases with super glue. I added some sticks and stones. It's simple but adds a lot to the model.
And here's a full 3'x3' board with nothing but trees and moss. Adding a campsite, some path ways, a river and there's plenty of detail for a forrest board.