I decided to add some background hills like I have in the past, but this time I only used the darker green for what I called the "nearest hills" before. I did this so when I paint the tree line I can leave gaps in it and you'll be able to see short patches of the hills in the distance.
After getting the hills painted, I went back with a medium fan brush this time and added a jagged edge across the top. I did this for a little texture and to enforce the look of a tree line rather than just a straight line.
Once, and if I'm successful at getting the foreground trees painted on, I'll come back to the exposed gaps in the tree line and add color and texture to the hills as I've done before with acrylics so it looks more like this.
The next process will be what is called "Blocking" in the trees. This is done with acrylics and will be very roughly painted patches of the close colors. In this case, greens and browns for the rough shape and color of the trees.
This is the basic idea of "Blocking".