As I stated above, the Preamble states that the purpose of the Constitution is to create a more perfect Union, which is an explcit acknowledgement that a "less perfect" union (that created by the "Articles of Confederation and perpetual Union") was already in existence.
From the perspective of the Constitution, the union of states did already exist, and was taken for granted. The Constitutional Convention was convened by the union of states in order to correct the defects of the Articles of Confederation (See Federalist 40), and to define a new relationship between Union and States. In that sense "the Union" really did create the states, at least insofar as they're defined by the Constitution.
But how does that explain North Carolina and Rhode Island whow were both in substance and in name, out of the Union after the Constitution had already been in operation?North Carolina didn't ratify the Constitution for nearly nine months after it was in operation and Rhode Island for full fifteen months. If the union created the states, now can they explain this one?
This is in perfect accord with Federalist 38, which said that the states are to be considered sovereign with regard to their ratification of the Constitution. The basic idea is that any state that did not ratify the Constitution was free to go its own way. It certainly wouldn't do to coerce a ratification from the states. However, once a state did ratify, it was explicitly bound to be part of the Union.
IOW, it's quite easy to explain.