Royal Icing is that sweet colorful piped icing that dries hard and makes sugar cookies look so good.
The original Royal Icing was made with egg whites and powdered (confectioners) sugar however with so many cases of crazy illnesses being attributed to raw eggs using meringue powder is the best way to go. Meringue Powder can sometimes be found in grocery stores during "cookie baking season" and its almost always in foodie oriented stores such as Whole Foods. Craft stores with a cake decorating section often sell meringue powder as well.
6 Tablespoons Water
The easiest way to prepare royal icing is to combine all of the ingredients in a stand mixer at the lowest speed using a padle attachment.
I have found that adding the sugar and merignue powder first, allowing those dry ingredients to combine and distribute works best. Add the water a little at a time, stopping the mixer and scraping down the sides of the bowl from time to time.
This will produce a sweet but fairly flavorless white icing that is ready to be piped.
Vanilla essense (extract) or any other flavoring can be added. Its often a nice surprise to add some peppermint extract to the royal icing that will be used to decorate candy cane shaped cookies.
Food coloring can be added as well.