So I had to study and try to figure it out on my own. :) Here is what I came up with.
Step 1: Tools
Green royal icing in a piping bag, with...
Grass piping tip.
Powdered sugar for snow.
Brown royal icing for tree trunks.
Toothpick to clean up excess icing.
Step 2: Begin your trees.
I felt most comfortable with turning the cookie upside down and piping the tree-needles up. That allowed them to lay flat and gave me space to move down the tree from there.
You want to have your royal icing as thick as the recipe makes it to be. Don't water it down. In order to keep the shape of the tree, the icing will have to be stiff enough to hold it's shape once you're done piping it.
Gradually move your way down the tree, layering your piping. Don't leave any gaps and make sure to just pipe naturally. It's okay to have imperfections because we are all from nature!
To clean up any edges with icing coming off the cookie face, just press down with a toothpick as if you are cutting the excess off. This will let it dry as it needs to later, with minimal crumbs from broken icing later!