Royal icing is great for decorating cookies because it dries hard, so you don't have to worry about your all of your hard work getting messed up en route to its intended recipient. And by using a couple of different consistencies of icing, you can get some lovely effects.
Egg whites are necessary for the hard finish of this icing, but meringue powder is a good substitute for anyone concerned about Salmonella (like me). Any craft store that has a cake decorating section will almost certainly carry meringue powder. And while you're picking up your meringue powder, you'll want to grab some concentrated gel food coloring. Just a tiny amount of gel will give you bright vibrant colors without messing with your icing consistency.
Royal Icing using egg whites (makes about 2 1/2 cups of icing)
2 large egg whites
1 tsp fresh lemon juice
3 1/4 cups confectioners (powdered or icing) sugar, sifted
In the bowl of your electric mixer (or with a hand mixer), beat the egg whites with the lemon juice. Add the sifted powdered sugar and beat on low speed until combined and smooth. Mix in food coloring, if desired. The icing needs to be used immediately or transferred to an airtight container as royal icing hardens when exposed to air. Cover with plastic wrap when not in use. (I cover with a damp paper towel). Rebeat before using again.
Royal Icing using meringue powder (makes about 1 1/2 cups of icing)
2 cups confectioners' (powdered sugar) sifted
1 1/2 Tblsp meringue powder
1/4 tsp almond extract (optional)
¼ cup warm water
In the bowl of your electric mixer (or with a hand mixer), beat the confectioners' sugar and meringue powder until combined. Add the water and beat on medium to high speed until very glossy and stiff peaks form (5-7 minutes). If necessary, to get the right consistency, add more powdered sugar or water. Mix in food coloring, if desired.
For the right consistency for piping the outline, lift the beaters and look for a ribbon of icing falls unbroken from the beaters, but remains on the top of the surface of the icing in the bowl.
Transfer your icing to a pastry bag fitted with a #2 round tip. Outline the shape of your cookie. Remember to hold the tip a couple of inches off the surface of the cookies to get a smooth, even line. (and if you don't want squiggles in your lines, don't let your kids watch and bump you.)
I like to keep a little bowl of water handy for touch ups. I dip my finger in the water and smooth over the sharp points left behind when I lift my pastry tip. It's also good to use that wet finger to gently push down on any icing that is not in direct contact with the cookie.
After your outlines have hardened, it's time to flood the cookies with icing. For this, you'll need icing that's the consistency of heavy cream. It needs to flow easily, but not be so watery that you can see the cookie through the icing. You add more water to the leftover outlining icing, or mix up a new batch.
Also, decide in advance whether or not you want your outline icing and your flood icing to make or be contrasting.
I apologize in advance for this next photo. I haven't quite figured out how to ice a cookie and take a blur-free photo at the same time.
Use a spoon to scoop some of your flow icing onto the cookie. Gently spread the icing around with the back of a spoon.