Once the cookies are cooled, attach a red M&M, with some melted chocolate, to the cookie.
Next, pipe two small dots of melted chocolate to make eyes.
Then pipe two lines above the eyes. After that make three smaller lines on your larger line. Make the smaller line have a slight angle. You have just created an antler.
This is probably the easiest decorating you could do on a cookie, and it is just about the cutest cookies for the holiday season (at least in my opinion).