So, here is how I do it.
1) I start with a box of Scenic Express Super Trees.
2) I spray painted all the Super Trees primer gray. I never thought much about this originally, but my friend Steve said look at the trees in the forest. Do you see many brown trunks? They are more grayish in color. Hey, I'm colorblind. Who am I to argue?
3) For the next step, you can use two different products. I chose to use a cheap extra hold hairspray. I believe I picked this up from the Dollar Tree Store. This seems to work well for fall trees with less leaves. For thicker foliage, you could use a spray or liquid glue (dipping the super tree in it first).
4) I pull out a tree. Usually, I have to trim it down to make it work for N Scale. I like the taller trees though.
5) I spray with the cheap hair spray and roll the tree around in the foliage. I like Woodland Scenics Coarse Turf. I then take more and sprinkle on top. Hit with the hair spray again, and keep adding foliage.
6) The finished product. It seems to work out well for my Fall trees. For thicker trees, I would definitely encourage using a liquid glue and dipping the super trees in that first.