Place the tube inside the bag and push or shake it down so its tip pokes out of the small opening at the bottom of the bag. (If the opening is too small, cut off a piece of the bag.) To keep your batter or icing from leaking out while you fill the bag, simply twist the bag right above the tube and push the twisted portion inside the tube.
Fold the top of the bag down to form a 3- to 4-inch cuff.
Slip one hand underneath the cuff while you fill the bag.
After the bag is filled, unfold the cuff and make an accordion pleat.