I found an easy way to do the trees, buy them from a hobby shop

Actually half joking, I bought a bunch of little trees from a shop.
My first attempt at making woods, I just stuck trees to a large base. I found that makes it ackward to move a unit through a wood.
Made up some plaster mix, which I poured on to a flat surface. So the blob of plaster was about 3 to 4 inches around (note: odd shapes look better). Imbedded 3 little trees in the mix and let it set. When it was dry painted and flocked.
I did this about 6 times
Got some cork board ( you can use craftwood) cut the shapes for the woods base, again odd shapes, not perfectly round. Again painted and flocked.
It wasn't that hard and I'm pretty happy with them.
Having the bunch of trees based in the plaster gives them some stability and makes them easy to move around.