Will harden unlike other Royal Icings
You want to start out by making your icing as stiff as possible. Whip it for a long time so it gets lots of air in it.Then, scoop a little into a small bowl, and tint it with your icing colors.
Finally, thin it down to the consistency you need.
If you are making a gingerbread house, you probably want it to be very stiff so that it can hold the structure together. Same goes for piping roses or any other kind of flower.
If you are making a royal icing transfer or decorating cookies, it’s nice to have 2 different consistencies: one that’s pretty thin for flooding (or filling in), and one that’s a little stiffer for outlining.
The flood icing will sort of self-level. In other words, when you pipe it out, it will naturally spread and smooth over perfectly flat. The outline consistency icing will hold the flood icing within its barriers, so it doesn’t spill over the sides of the cookie. You can also use it for accents here and there.