Now that we've made some cookies, let's go over the icing you can use to cover them. As in all aspects of life, there is more than one way to do this - you can use many mediums to ice your cookies. The ones I'm familiar with are Glaze and Royal Icing or a bastardization of the two that I am growing to like more and more. The most important thing when using any of these recipes is getting the correct consistency for your needs. If your icing is too thin, it will spread all over and be a big mess; if it's too thick it will give you arthritis, break a tooth and generally not work out very well. I'm going to put some links in this post that are very helpful if you're interested in learning more.
Glaze icing does not dry as hard as Royal Icing but many prefer the taste. I don't prefer using glaze just because it handles different than Royal, which I am more accustomed to. I also find that cookies break more easily and you have to be more careful about stacking and such or the icing cracks. If you want a Glaze recipe, look here. You can also poke around her blog and see some of the amazing work this woman does with glaze icing.
Many cookie decorators prefer to use Royal Icing because it holds detail better (See that sheep above with its fluffy coat? That fluff only holds with a thick Royal...the head and feet are thinned down). Royal Icing recipes are all over the internet, I recommend adding some clear vanilla to improve the taste. I also like to add some white food coloring to the big batch of icing. One of the downfalls of Royal (for me) is that it separates easily. Many times in cookie decorating you must let a step dry for an hour or more and come back to it later. I don't know how many times I've gone back later to do details and my icing has started to separate. It must be emptied out of the pastry bag or bottle and stirred to reincorporate, then you have to let it sit for a while to let the air bubbles come to the surface before you can re-bag or bottle it and use it again.
Air bubbles, what do you mean air bubbles? This is a measuring cup with icing that has sat for about 10 minutes. All those bubbles were created from mixing the food coloring (and lots of air!) into the icing. It is best to get rid of those bad boys before they're sitting on your cookie, just waiting to make something ugly happen. Run your spatula or knife across the top to get rid of as many of these as you can. Read more here.
The bastardization I mentioned comes with the recipe for Royal Glaze which combines the best of both worlds. It has most of the characteristics of Royal Icing, but is softer when you bite into the cookie. It also tends to hold up better during decorating - meaning that I don't have to worry so much about separation and remixing my icing during a decorating session. I loosely follow the above recipe but do not use butter flavoring (don't care for the taste) and I tend to use less water. I like a thicker icing that more resembles Royal than Glaze, then I thin it down with water as needed. If you add the full amount of water right at the beginning your whole batch will be a little watery (which is fine if that's what you like!).
Both Royal Icing and Royal Glaze call for meringue powder which can be purchased online or in stores like Michael's in the baking/Wilton aisle. If you buy at Michael's I recommend you use a coupon, this stuff gets expensive. Royal Glaze also calls for glycerin - this is food grade, not cosmetic grade and can also be found at Michael's in the Wilton section of the store. Those are the only two ingredients I can think of that won't be found at the average grocery store.
I use pastry bags and bottles with decorating tips, but that is not necessary. If you just want to try this out casually and don't want to invest much money, buy some freezer bags, reinforce a corner with tape and then cut a small hole in it. I read once that this woman only uses freezer bags, I can't verify that, but those cookies are beautiful.
When you make any of these recipes, it is important that the bowl of icing be covered at all times. If it isn't, a crust will develop on the top and when you try to remove icing from the bowl, the crust will crumble and fall into the good icing. These crumbles can clog tips and be a pest. I dampen a dish towel and set it over the bowl or bowls, checking often to make sure nothing is drying out.
Consistency is key with any of these recipes. Check out some of these posts for more information as these women have put some great information out there.
The Adventures of Sweet Sugarbelle - this is a great discussion on decorating basics
The Adventures of Sweet Sugarbelle - another discussion on consistency
Lila Loa - another point of view on consistency
There are several great blogs/websites out there with information on cookie decorating. I highly recommend checking out the two blogs above as well as www.karenscookies.net for tons of tips.