I tried, initially, to sprinkle the powdered sugar onto the trees before they had dried...BuUut...the powdered sugar got into the icing and absorbed lol. Best to wait for them to dry.
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. :)