Forgive my brevity: I'm afraid my words are required elsewhere these days, and there are only so many to go around.
Maybe some purple houses and orange leaves will help. I've found them quite useful myself.
Iced Sugar Cookies
3/4 cup unsalted butter, room temperature
3/4 cup sugar
1 teaspoon vanilla
2 2/3 cups cake flour (may require a bit more)
1 teaspoon baking powder
1 teaspoon salt
2 egg whites, room temperature
2 teaspoons lemon juice
3 cups icing sugar
1/4 teaspoon almond extract
In a large bowl, cream the butter and sugar until pale. Add the eggs and vanilla and beat until smooth.
In a separate bowl, whisk together the flour, baking powder and salt.
Add the dry ingredients to the wet ingredients and mix gently. If the dough is still very sticky, add 1 tablespoonful of flour at a time until rolling seems possible. Keep in mind that the dough will firm up when refrigerated, so err on the side of under-flouring.
Divide the dough in two, wrap each disc tightly in plastic wrap, and refrigerate for at least 1 hour.
Preheat oven to 350 degrees Fahrenheit and line baking sheets with parchment paper.
Roll the dough out on a floured surface to 1/4". Cut into shapes and carefully transfer the cookies to the baking sheets, leaving at least 1" all the way around.
Bake for 8-12 minutes or until cookies are lightly golden around the edges. Cool on a rack.
To prepare the frosting, beat the egg whites, lemon juice and almond extract until frothy. Slowly add the icing sugar, beating continuously, until the mixture comes together. Add extra icing sugar (or water), as needed, to thicken (or thin) the icing for piping (or flooding). Tint the frosting and decorate the cooled cookies as you wish.
Royal icing can be covered with plastic wrap and stored at room temperature while you decorate.
Source: Butter Cut-Out Cookies in Nigella Lawson's How To Be a Domestic Goddess, p. 212.