There is a lot to learn in the wide world of cookie decorating, but these basic tips should get you started.
Supplies
Cut-out sugar cookies
Royal Icing
Food coloring
Bowls and spoons
12-inch disposable decorating bags
Standard couplers
Decorating tips, sizes 2, 3, 4 and 5
Rubber bands
Scissors
Toothpicks
Small knife or spatula
Paper towels
Helpful Hint: You can find my go-to recipes for sugar cookies and royal icing below, and my go-to recipe. For food coloring on my blog, The Decorated Cookie.

Preparing and Coloring Icing
To test the consistency, take a spoonful of icing and turn it over the bowl. It should cling to the spoon before slowly falling back into the bowl. To thicken royal icing, add powdered sugar. To thin royal icing, add water.
Divide your icing into bowls and add a drop of your food coloring. Stir well. Continue to add drops of food coloring, one at a time, to achieve desired shade. Stir well. Assembling and Filling a Decorating Bag
Using a coupler with your decorating bag enables you to easily change decorating tips while decorating cookies.

To assemble a decorating bag with a coupler and tip, snip about 1/2 inch from the tip of the bag. Unscrew the ring from a coupler and insert the coupler into the bag, narrow end first.
The coupler should fit fully inside the decorating bag. Hold a decorating tip over the cut end of the bag and onto the coupler, and screw the ring back on.
Fold over the top of the bag to form a cuff, fill the bag with icing (about ½ cup to ¾ cup), twist the bag above the icing, and close tightly with a rubber band.
Piping Icing
Practice piping icing on a piece of wax or parchment paper.
To pipe, hold the bag in your dominant hand as though you were holding a glass of water. Rest your other hand on top of your dominant hand to steady the bag.

Squeeze with steady, consistent pressure to pipe the icing, letting the decorating tip hover about 1/4 inch above the surface. To stop, quickly press the decorating bag down, then pull away.
To decorate cookies by piping icing, first pipe the outline of your cookie design, let the icing set about 15 minutes, then pipe back and forth in a zig zag motion to fill the entire cookie with icing.

Flooding Icing
The "flooding" method of cookie decorating is similar to above, but by piping an outline to serve as a dam and then filling the cookie with a thinned icing, your cookies will have a smooth, finished look without showing the lines of icing from piping.
To flood cookies, pipe an outline on your cookies according to the instructions above, and let set about 15 minutes.
Spoon some of the same color icing in a bowl and add water, about 1/4 teaspoon at a time, stirring well after each addition, until the icing is of flooding consistency.
If you take a spoonful and pour the icing over the bowl, the icing should drizzle into the bowl and disappear in about ten seconds. When in doubt, err on the side of too thick rather than too thin and runny
Fill an empty decorating bag with the thinned icing, snip about 1/8 to 1/4 inch from the tip and loosely "pipe" the icing on the cookie. Let the icing flood to the piped outline, encouraging as needed with the back of a spoon or with a toothpick.
Let the icing dry very well, preferably overnight. When dry, add details on top of the flooded cookie with piped royal icing designs. (Or, if you prefer a different effect, add details when the icing is still wet, and they will sink and blend into the icing.)
Choosing a Decorating Tip
Here is a general guideline for choosing the right size decorating tip.
Size 2: Use this small, round tip to add details, such as smiley faces, dots and swirls, on top of a cookie flooded with icing.
Size 3: Use this tip to outline cookies and add details.
Size 4: This tip may also be used for outlining cookies or for filling a cookie in by piping back and forth.
Size 5: This large, round tip is used for filling in cookies by piping back and forth.
Helpful Cookie Decorating Hints
Practice piping on wax or parchment paper first.
Unclog decorating tips with a toothpick.
Keep lots of paper towels handy.
Leave cookies on a baking tray at room temperature overnight to dry well enough to handle and package.
Don't fret if you make a mistake. Just scrape the icing off of the cookie and start again!
