Whisk the egg whites in a stand mixer on medium speed until frothy. (You can also do this in a mixing bowl with a hand mixer.)
Add in almond extract or extract of your choice.
Add the powdered sugar 1 cup at a time on low speed. Make sure to scrape down the sides of the bowl periodically.
Once all the sugar has been combined, increase the speed of the mixer to high and whisk for 7 to 10 minutes. Stiff peaks should form and when you run a knife through the icing the path of the knife should hold.
Fill a piping bag with the icing and use it immediately for the best results. (see note)
Notes
If storing the icing, put it into an airtight container and lay a piece of plastic wrap over the top so that it's touching the icing. This will keep the air off the icing and prevent a crust from forming and the icing from drying out.