Pastry bags are a neat and easy way to frost desserts. Many recipes and baking tutorials call for a device called a coupler inside the pastry bag. It lets you switch the tips attached to the bag without having to emptying its contents first. If you have only one pastry tip and no coupler, improvise using the tools you have.
Drop a piping tip into the pastry bag and push it down as far it can go. Draw a small horizontal line on the outside of the pastry bag just above the opening of the piping tip inside the bag.
Remove the piping tip from the bag. Using scissors, trim off the tip of the pastry bag along the line you marked.
Push the piping tip down into the bag so that the opening of the tip protrudes through the hole in the bag.
Fold the top of the bag over to create a cuff. Slide your hand up under the cuff and hold the bag loosely in your hand.
Fill the pastry bag halfway with the frosting, then unfold the cuff. Pinch the bag closed just above the frosting, and twist it shut to secure the closure.
Gently work the icing down until it comes through the opening of the piping tip, and frost your desserts to your heart's content.