Wipe down the bowl of a stand mixer, the paddle attachment and any spatulas you will be using with white vinegar. This is to remove any grease from the surfaces of the utensils that will come into contact with the royal icing.
In the bowl of a stand mixer, using the paddle attachment, mix the powdered sugar, salt and meringue powder on low speed to fully combine.
In a measuring cup, stir together the water, glycerin and extract.
Pour the mixture into the mixing bowl and beat on low speed until the wet and dry ingredients start to come together.
Scrape down the sides of the bowl and then beat on medium speed for about 3 to 5 minutes. The mixture should be thick and bright white.
Transfer the royal icing to an airtight container, preferably a glass container. If you use plastic containers make sure they have not been used to store anything else previously. The grease residue from foods will break the icing down.
Store them in the refrigerator or on the counter at room temperature.
Royal icing will stay good for a couple of weeks. If separation occurs, stirring will help to bring it back together.