Measure exactly 4 cups of powdered sugar and use a fine-mesh sieve to sift it directly into a very large mixing bowl (or stand mixer bowl). Add the 4 tablespoons of meringue powder. Whisk the dry ingredients thoroughly for 30 seconds until uniformly combined.

In a separate cup, combine the 1/2 cup of warm water, 1 teaspoon of clear extract, and 1 teaspoon of corn syrup (if using). Stir these wet ingredients gently.

Using the whisk attachment on the stand mixer (or a strong hand mixer), start mixing on the absolute lowest speed. Slowly stream the wet mixture into the dry ingredients. Once combined, scrape down the bowl.

Increase the mixer speed to medium-low and beat the mixture continuously for 5 to 7 full minutes. The icing should become opaque white, thick, and hold stiff peaks. This is the 'Stiff Consistency.'

If you require 'Outlining Consistency' (for piping borders), remove about 1/4 to 1/3 of the stiff icing now, place it in a separate bowl, and cover it tightly with damp plastic wrap to prevent it from hardening.

To create 'Flooding Consistency' (for filling cookies), add 1 tablespoon of warm water to the remaining thick icing in the mixing bowl. Mix on low speed for 30 seconds. Test the consistency by scooping a spoonful and letting it drizzle back into the bowl.

The goal is '10-Second Icing': the line drawn by the drizzle should disappear completely and smoothly into the surface of the icing in 8 to 12 seconds. If it's too slow, add water 1 teaspoon at a time. If it's too fast, add powdered sugar 1 teaspoon at a time.

Divide the finished consistencies into smaller bowls and color them using gel food coloring mixed in with a toothpick. Transfer the icing into piping bags (fitted with small round tips for outlining, or just snipped tips for flooding).

Store leftover royal icing in an airtight container at room temperature for up to 3 days, or refrigerate for up to 2 weeks. Ensure the surface is covered directly with plastic wrap to prevent a crust from forming.

Allow decorated cookies to dry completely at room temperature for a minimum of 6 to 8 hours, or preferably overnight, before handling, stacking, or packaging.