just until the milk is absorbed, mixing gently with a fork. you should be able to gently press the dough into a ball. mix the dough as little as possible: you don't want to cream the lumps of butter into the flour. a crust without lumps of butter will be dense, not flaky. note that humidity will affect how much liquid the flour will absorb.
split the dough into two parts; 2/3 and 1/3 (2/3 for the bottom and 1/3 for the top). pat them into balls, flattening them slightly, and wrap them in plastic wrap. the dough needs to rest in the refrigerator for at least 30 minutes. overnight is 