In a large bowl using a hand mixer or a stand mixer fitted with a paddle attachment, beat the powdered sugar, meringue powder, and water together on medium-low speed for about 5 minutes, or until it is not shiny. At this point, the icing will likely still be too stiff for piping. Add a tiny bit of additional water and mix until incorporated. Continue to do so until your icing is the right consistency for piping the outline... not too stiff, but not runny either. That's helpful, right?! It's better to be too thick than too thin. It should come off your beater and smooth out in 10-15 seconds. If you're unsure, put some in your decorating bag and give it a try. If it's difficult to squeeze or breaks apart as you pipe out a sample line, it needs more water. Mine was probably a bit too thick still on these trees (you can see where I stopped and started since it was so stiff).