I get asked a lot what kind of icing I use. This is my favorite.
I have tried a lot of icing recipes but I think that Royal icing works the best because it hardens enough so that you can stack the cookies and the icing won't get ruined. The downside to royal icing is that it dries out after a few days so I've found that it's better to make the cookies within a day or two before you take them somewhere. You can freeze the cookies after you have decorated them. They are still yummy and taste fresh!
Here is the link to the royal icing recipe:
I usually double the recipe and use more icing in each color than I might need just so I don't have to worry about matching colors if I have to make another batch. Just something I've learned through many trials and errors.
You can find the meringue powder at walmart in the cake decorating section with the crafts. They also have it at hobby lobby.
Hope this helps!