When cookies cool, make Royal Icing:
In mixer beat egg whites on high for about 6-8 minutes until egg whites form thick peaks. It is critical for you to be certain to beat the eggs long enough in order for this recipe to work.
Beat in lemon extract.
Stir in 4 cups of powdered sugar (by hand) and then, once mostly incorporated, beat on high until thickened. You may add additional powdered sugar for thicker results.
Separate out small amounts of the frosting and color with food color as desired. Put frosting into sandwich baggies or piping bags, cut off very small piece of lower corner of bag and squeeze frosting out onto cookie to decorate.
Tip: This frosting hardens fairly quickly once it is on the cookie, so I recommend icing about 10 cookies with the white frosting at a time until you've iced all of them; then go back and decorate with the colored frosting.
Royal Icing recipe is from Allrecipes.com