Place half of the powdered sugar and the meringue powder in the bowl of an electric mixer. Whisk together well.
Turn on mixer (use whip attachment) and, while motor is running, slowly stream in the water. Mix until everything is incorporated.
Turn mixer to high speed and whip until stiff peaks form.
Add flavorings and mix well.
Change to paddle attachment for stand mixer If using a hand mixer, use the same beaters you were using before.
Add remaining powdered sugar and shortening and whip for 2-3 minutes more.
Divide the frosting into 3 sections- 1/2 Green, 1/4 White, 1/4 Red
Decorating the cookies:
Use tip # 5 pipe a line of green frosting around outside edge of cookie, flood cookie with green frosting. Let dry completely. Pipe decor on tree with remaining tip # 2 or 3.
Notes
Yield depends size and shape of tree cookie cutter.