To start, you'll need meringue powder. Ateco is my favorite...I think I've tried them all. :) It is a little hard to find, so any meringue powder will work. Follow the instructions for Royal Icing that come with your meringue powder. The most important step: sift the powdered sugar! Believe me, I've tried to shortcut this step...it doesn't work. (My favorite powdered sugars are Domino and C & H, but as you can see from the picture, any will do. Imperial is easier to find in the 1 pound boxes.)
No matter what recipe you're using, you will beat the icing until it comes to a stiff peak. When the beater is pulled from the icing, the peak should stand steady. You can also pull the beater off the mixer and hold the whisk attachment upright. The peak should stand, even if you wiggle it.
At this point, you can add a few drops of flavoring if desired. Remember to use a clear flavor as brown will tint the icing.
Pastry bags...I can't say enough about disposable decorating bags! They are great. The 100 bag box is a perfect use for the weekly 40% off Michael's coupon. :) Use the bags with the white couplers to easily change out the tips with the same bag of icing.
Once you've filled the bag with icing (I generally only fill half to 2/3 full), twist the top and stand upright in a glass. If your bag will be sitting awhile unused, place a dampened paper towel in the bottom. Just be sure to squeeze a little icing out first before going to the cookie. The tip might be too wet and you'll have a water blotch.