[header] How to make a cake decorating bag [title] Find a flat surface, such as a large, clean cutting board, to work on. [step] You can use the decorating bag immediately after making it, so keep your frosting nearby. [title] Tear a large, rectangular piece of parchment (baking) paper off the roll. 
[step] There is no specific size that it must be, but you should aim to have a piece that is approximately 12 inches (30. 5 cm) high by 14 to 18 inches (35.6 to 45.7 cm) long.