Use a glass of metal bowl (very clean and dry with no oily residue)
Start to whip 1/2 cup of water and 1 cup of our Meringue mix using a countertop mixer with a whisk attachment.
This will be done for 9-12 minutes (until there are no stiff peaks). While is whips, scrape the bowl as needed and avoid overbeating and stop when you get stiff peaks.
Gradually mix in 3 cups of powdered sugar on low until incorporated before using a spoon to finish it off and avoid air bubbles.
Add in a tsp of water at a time, using a spoon to mix it in, until you've gotten the royal icing texture you desire (needs may change based on what you're using it for).