I've taught quite a few people to decorate cookies in my day. At least like.... seven people. So I consider myself to be somewhat of an expert beginning cookie decorator watcher. And I would say that perhaps the single most frustrating aspect of decorating for a beginner is getting consistency right.
And the second would be watching me pipe an outline and then trying it themselves only to discover that their perfect outline magically turned into a thin, squiggly outline instead. You can read all about icing consistency in THIS POST. And once you've got that mastered, or if you like to live in denial... we can talk about straight NON-SQUIGGLY outlines. At the risk of being incredibly annoying, I will say that first you need to have the right consistency of icing. It's got to be thick, or this just won't work. Second, don't touch the cookie! The tip should only touch the cookie when you start, stop, and make a corner. Other than that, your goal is to lay down the icing as if you were outlining the cookie with a piece of yarn.
Watch your speed. If you find that the icing is coming out too fast, you can make it thicker or just stop squeezing so hard you Olympic Champion Icing Squeezer you! If the icing is breaking or being stretched thin, you need to squeeze a little harder or thin the icing a bit. I like to keep pressure on the back of the bag with bag clips so I don't have to squeeze as hard.
And once you've figured out how to make the lines straight... I've got one more trick to keep them from going crooked. Anita from Sweet Hope Cookies shared this gem with me. Grab some striped scrapbook paper next time you're at a craft store. Or print some off your computer. Slip it in a sheet protector and place your cookie on top. Use the lines on either side to help keep you on track. Just like above, touch the cookie when you start and stop on each side of the line and then fill in any areas in the middle.
Can you save over-mixed icing?
Quick and Easy tip for filtering icing so it doesn't clog your favorite tips.
My favorite tips.
Cookie Decorating Quickstart for Beginners