First things first.  A pastry bag can usually be purchased at a craft store, some grocers, and baking supply stores. Traditionally pastry bags were made from an oiled  linen type material but today the home cook can get them in parchment or plastic and polyester. Essentially a pastry bag is a cone shape that is fitted with a tip that allow the user to apply a frosting to cakes,cookie,sand pastries in a decorative pattern.  The bags of today are often disposable paper,but for the cook who may want to decorate more often the reusable varieties are preferred.

The frosting mix usually includes egg whites to aid in stiffening as the frosting sets. Today most cooks will use a powdered egg white substitute in making the frosting to avoid concerns about salmonella contamination. 

Tips can be purchased singularly or in kits with a variety of common sizes and designs.  Some of these are stars, ribbons,and leaf tips.These three designs can create many decorative effects.

You don't have to rush out and purchase a pastry bag, however.  You can make your own that will serve the purpose nicely. I prefer to use parchment paper to make my own bags.  This way I can make several, each with a different color frosting, making decorating quick and easy.  You can also use a zipper bag, the kind you store food in, with a tiny tip cut off of the corner. Either of these will give you a useful tool with which to decorate your baked goods.

To use the bag of choice, you must first insert the tip.  Purchased bags and tips require a collar or and ring called a coupler to lock the tip into place.  These will come in the tip or can be purchased alone where you buy your bags. You can use a zipper bag without a tip,however.  You will get strings of frosting that can be used to write words like Happy Birthday, or you can create pearls.  Pearls are small dots of frosting placed close together to form what looks like a string of pearls.A little experimentation on a sheet of wax paper will serve up a variety of options.

Once you have fitted your bag with the tip of your choice it is time to fill it with frosting.

To fill a bag,hold it in your hand loosely and fold the wider top part down over your fist.  You want to fold it about 1/3 to 1/2 of the way down and cup the remaining bottom firmly but don't squeeze it.  Use a a spatula or round tipped knife to pile frosting into the bag. You will scoop up a quantity of frosting and put it into the bag so that it is as near the tip as possible.  Draw the spatula or knife back out of the bag,scraping the side of the bag as you do so that the frosting remains inside.  Repeat this until you fill the portion you are holding.  You should not fill the bag more than 2/3 full.  Once you fill the bag, pull it back up and twist the top closed.

You will then grasp the bag between your thumb and first finger, holding it so that the twisted area rests in the crook between these two fingers and the remainder of your hand,the palm and fingers.  You will use this hand to gently squeeze the bag and the other to guide it. 

This technique takes a little practice to achieve even results,but with time you will be decorating like a professional pastry chef. You can practice your technique using zipper bags with a small corner cut off to pipe filling into deviled eggs, pile mashed potatoes into the shells of a baked potato to make twice-baked potatoes, or onto platters to create a pleasing presentation. You could even try piping peanut butter or cheese onto crackers. This will help you learn how much pressure to put onto the bag.

I hope you will try using a pastry bag as you set about making cookies for your family or a cake for that next special occasion.  With a little practice you can be decorating like a pro.