I don't own a piping or pastry bag mainly because I hate cleaning the reusable bags, and I don't want to buy plastic bags specially designed for pastry piping.  So, how do I frost cookies without a piping bag? I have found piping bag alternatives for frosting cakes or cookies.  I even use these alternative piping bags to pipe cream puffs, meringues or pavlova nests.  

Parchment paper

If you don't have freezer bags, and you are desperate, you can use parchment paper as a piping bag alternative.  Shape your parchment paper into a cone with a very tiny tip opening.  Fill the parchment paper with your filling.  Roll down the excess paper at the opening so that no filling exudes through the top.  Cut the tip opening to the size needed.

After a while, the parchment paper's tip will increase in size.  The filling also tends to dry up if not used immediately.

Plastic bags

1 gallon or 1 quart disposable freezer bags are great to use as piping bags.  They are thick, easy to use and when you're done, just throw them away.  I love using these, especially when working with sugary fillings like icing and meringue.

Tips.

Type of bags.

It is important to use the thicker, FREEZER bags. The freezer bags can withstand more pressure.  Cheap plastic storage bags may split at the sides and you'll end up with a mess.

Using piping tips.

You can use piping tips with the FREEZER bag.  You need to place your tip in first before adding the filling.  I have even used a disposable freezer bag with a tip coupler. Cut a small hole at one of the corners and place your piping tip through it.  The hole should be small enough that the plastic bag has to really stretch to securely close around the piping tip or coupler.  If it is not tight enough, your filling will ooze out between the plastic bag and tip.

Angle the bag.

When adding your filling, open the bag to form a conical shape, you can even place this in a jar, folding the opening around the jar which results in less mess.  Spoon your filling in so that most of the filling is at one corner of the bag.  

Remove air.

Flatten the bag to remove the air before sealing the bag shut.