I've seen some pictures on here where the icing looks so smooth. How do you do that?

Might be fondant.....or do a search on VIVA for the paper towel/typing paper method. Now paint roller also!

Maybe royal icing--there is a tutorial.

If it's royal you're talking about, it is watered down to be able to "flood" a cookie.

Here's the tutorial and recipe by Helen (aka Antonia74):

http://www.cakecentral.com/article54-How-To-Bake--Decorate-Cookies.html