Royal icing is a smooth hard-drying icing. It works great for icing cookies and decorating cakes. Royal icing is traditionally made with egg white but personally I am a little apprehensive about eating raw egg white due to salmonella. So I substituted it with meringue powder. This is the Wilton meringue powder that I use. You should be able to get it at your local baking supply store.
3 tablespoons meringue powder
4 cups icing sugar
6 tablespoon warm water
1/2 teaspoon vanilla or lemon juice
Mix all ingredients and beat with a mixer till stiff peaks form. It takes about 5 – 7 minutes. If necessary, add more powdered sugar or water to get the right consistency. This can be used to pipe a border around the cookie. Then to “flood” the center of the cookie, you need to thin it down by adding 1/2 tsp of water at a time till you get the right consistency. If the royal icing is not used immediately, keep it in an airtight container because royal icing hardens when exposed to air.