Generally, it is best to assemble a dumpling immediately after you roll out each wrapper. This makes folding much easier, because the dough hasn’t had a chance to dry out. It will be pliable and easy to work with, as well as self-sealing (you don’t need water to stick the dough together).  You can prepare the dough several hours in advance, and just leave it covered in an airtight container so it doesn’t dry out. If you refrigerate the dough ball in an airtight container or reusable bag overnight, you can assemble the wrappers the following day, but they won’t be as fresh. Rolling out the wrappers ahead of time is also possible (see refrigerating/freezing instructions below), but this can be more of a hassle, because you will have to take steps to prevent them from sticking to each other. It’s much better to assemble the dumplings all at once, and then freeze the dumplings, not the wrappers. Then you can boil, steam, or pan-fry them whenever you like (they don’t have to thaw before cooking).