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!
Step 3: If Your Tip Isn't So Grassy...
In order to fix this effect on the cookie, just use your toothpick and scrape all over. That will take it from the totally controlled look, to the freestyle-wavy tree effect. Maybe you prefer this, I'm not sure. You can decide!
Then leave them to dry overnight. :)
Step 4: Let It Snow!
Then you can sprinkle the "snow" on them before packaging. These work very well and even though they are simple, they impress people! :) If you want, you can pick ornamental sprinkles and stars to put on your trees. :)