Using a Pastry Bag
Using a pastry bag may seem intimidating if you've never worked with one, but with just a little practice you will find that it is a surprisingly easy task.
You'll need a pastry bag, a plastic 2-piece coupler, and a metal or plastic pastry tip. Most brand new pastry bags need to have their openings trimmed so the coupler will fit, sticking about 1/2 inch out of the opening. When you do this, be sure to trim off only a very small amount. Then try the tube to see if it fits. If the opening is still too small, trim off another very narrow sliver. Keep doing this until the coupler fits into the bag with about 1/3 of the coupler sticking out of the opening in the bag. If you cut off too much, the coupler will push all the way through, and the bag will not work for piping, so be careful.
You can buy either disposable bags or reusable bags. The reusable bags are washable and will last for many uses. When you use a large pastry tip you don’t need to use the coupler. Insert the pastry tube into the pastry bag, so that about 1/2 inch of the tube is sticking out of the bag.
Place the bag (with the tip inserted) into a 2-cup measuring cup or a large iced tea glass for support. Fold the top of the bag halfway down. Use a spoon or rubber spatula to fill the bag with marshmallow batter or marshmallow fluff. When you have filled the bag halfway, remove it from the glass or measuring cup, unfold the top of the bag, and hold it firmly in your right hand (left if you are left-handed) in the curve between your index finger and your thumb, just above the contents of the bag. Gently twist the bag so that any air comes out and the contents of the bag start to emerge from the tip.
Keep your hand around the bag where it's twisted, and be sure to keep twisting the bag gently as you pipe so that the contents of the bag stays down at the tip. Hold the bag this way with your right hand (or with your left hand if you are left-handed) and place the tip onto the parchment-lined baking sheet. Using your left hand to steady the bag (or right hand if you are left-handed), gently squeeze the bag as you hold it at the twist. As the contents of the bag comes out of the tip, raise the tip slightly from the parchment, releasing pressure from the bag and pressing down very slightly with the tip when the piped item is the size you want it to be.
Practice this a few times, returning the piped dough to the mixing bowl, until you feel comfortable with the process. Then pipe the marshmallow onto a prepared surface, or pipe the marshmallow fluff onto a dessert. When the bag becomes empty, flatten it on your work surface and use a spatula on the outside of the bag to push the marshmallow that is clinging to the inside of the bag down to the tip. Then open the bag, place it in the glass or measuring cup, fold down the top, and repeat the filling and piping process, until all of the marshmallow or fluff has been used up. Wash the bag and the tip in warm, soapy water. Be sure to dry the tip thoroughly or it may rust.