Ensure your mixing bowl and whisk (or stand mixer bowl and whisk attachment) are completely clean and grease-free. Separate the egg whites carefully so no yolk gets in. Let egg whites sit at room temperature for about 15–20 minutes if they were refrigerated.
3 large egg whites
Add the egg whites and lemon juice (or cream of tartar) to the mixing bowl. With a hand mixer or stand mixer on medium speed, whisk until the mixture looks frothy and slightly lightened, about 30–60 seconds. You’re just building a little structure, not full meringue.
3 large egg whites, 1 teaspoon fresh lemon juice
Turn the mixer to low and begin adding the sifted powdered sugar about 1/2 cup at a time, letting each addition fully incorporate before adding more. Scrape down the sides of the bowl once or twice to ensure everything mixes evenly and to avoid dry pockets of sugar.
4 cups powdered sugar
After all the powdered sugar has been added, increase the mixer speed to medium-high and beat for 3–5 minutes, until the icing is thick, bright white, and glossy, similar to marshmallow fluff. When you lift the whisk, the icing should fall in a thick ribbon and hold soft-to-medium peaks. This is your stiff-consistency royal icing base.
3 large egg whites, 4 cups powdered sugar
Turn the mixer down to low. Add the vanilla extract (and almond extract, if using) along with a pinch of fine sea salt. Mix just until combined to avoid incorporating extra air, which can cause bubbles in your finished cookies.
1 teaspoon vanilla extract, 1 pinch fine sea salt, 1 teaspoon almond extract
For outlining and flooding, thin portions of the stiff icing with water. Transfer some stiff icing to a smaller bowl. Add water 1/2 teaspoon at a time, stirring thoroughly after each addition. For outline (medium) consistency, aim for a ribbon of icing that falls off a spoon and disappears back into the bowl in about 15–20 seconds. For flood (thin) consistency, the ribbon should disappear in about 8–10 seconds. Go slowly; if you over-thin, you’ll need more powdered sugar to fix it.
1-3 tablespoons water
Divide the icing into separate bowls for different colors. Add gel food coloring a tiny amount at a time (the tip of a toothpick is usually enough to start). Stir well, then let the icing sit for 5–10 minutes; the color will deepen as it rests. Adjust with more gel color as needed.
gel food coloring
Spoon your various icing consistencies and colors into piping bags fitted with small round tips (such as Wilton #1–3). If you don’t have piping bags, use zip-top bags and snip a very small hole in one corner. Twist and secure the tops of the bags to prevent icing from oozing out.
Outline your cookies with medium (piping) consistency icing and let the outlines set for a few minutes. Flood the centers with thinner icing, using a toothpick or scribe tool to nudge icing into corners and pop any air bubbles. Let the decorated cookies dry uncovered at room temperature until surface-dry (about 2–3 hours) and fully set for stacking, 8–12 hours or overnight, depending on humidity.
While working, keep any bowls of icing covered with plastic wrap pressed directly onto the surface, or with a damp paper towel under the lid, to prevent crusting. If icing thickens as it sits, stir gently and add a drop or two of water as needed to restore the desired consistency.