I find making homemade trees to be one of the most enjoyable projects to do in creating a layout. And, the trees greatly enhance the appearance of the layout IMO.
The basic structures I use are natural and free of charge. They consist of looking for symmetrical tree branches when going for walks. Then, I do what John says he does: I use polyfiber glued on to the branches, and ground foam glued on to the polyfiber. The polyfiber and ground foam are products from Woodland Scenics or Scenic Express.
Some of my homemade trees are simply the tops of tall weeds.
Below are a few photos of my homemade trees:
Arnold
