Small and Disposable bags work best for detail work.
Large bags are great for piping sheets of icing and foods textured like mashed potatoes.
If you are fitting a plastic bag with a decorating tube, start by cutting about 0.25 in. off the tip. Put decorating tube into bag so that the small opening of the tube is exposed. (If the opening needs to be widened, remove the tube and cut a bit more.)
Before filling, fold the wide opening of the bag outward, to form a wide cuff. Keep your writing hand free, and place the other hand around the bag under the cuff, as you would hold a cup, to keep it open while being filled. Fill up to 75% of the bag using a spatula. Press the filling against the sides of the bag to keep out air pockets. Push up against the cuff to make it smaller as the bag fills. Use your writing hand to unfold the cuff and twist the top to close the opening. Hold the twisted part snugly in the crotch between your thumb and index finger of your other hand, and apply pressure to start moving the contents through the tip until you get an even flow. Guide the tip with your writing hand.
Squeeze and twist. For best control and an even flow, keep the bag tight by twisting the top frequently, as you squeeze out the ingredients.
With very small bags, and when using very small amounts of filling, you'll only be using your writing hand for squeezing the bag and guiding the tip.
To clean, hand wash in warm soapy water. Air dry.