If this recipe required a lot of piping that would also need to be pretty accurate, I would suggest using a piping bag with a tip. However, because this is a simple recipe and intended to be more 'fun' than 'accurate', you can go the simple route. The simple route entails filling a zip top bake about half way full. Then, simply snip off one of the corners (1/4-1/2" or so) and use that open hole to pipe your chocolate.