The royal icing recipe I use contains only two ingredients: icing sugar and egg whites.
Quantities are adjusted slightly according to use – generally anything from “stiff peak” for borders, to “soft peak” for string work. I always add whites to sugar, never the other way around. In most cases, only a drop or two of whites is required to adjust the icing. I prefer mixing by hand in small quantities, which allows me to develop a sense for the correct consistency.
Proper consistency is key.