What was required was
a. the assent of 3/4s of the other states if separation was attained by amendment, or
b. federal legislation, or
c. a successful court case between the state and the federal government. That last would have required a grievance.
Of course, A. and B. themselves are worthy of grievance because they were/are impossible limitations to overcome.
Nowhere in the Constitution is there any mention of the union of the states being permanent. This was not an oversight by any means. Indeed, when New York, Rhode Island, and Virginia ratified the Constitution, they specifically stated that they reserved the right to resume the governmental powers granted to the United States. Their claim to the right of secession was understood and agreed to by the other ratifiers, including George Washington, who presided over the Constitutional Convention and was also a delegate from Virginia. In his book Life of Webster Sen. Henry Cabot Lodge writes, “It is safe to say that there was not a man in the country, from Washington and Hamilton to Clinton and Mason, who did not regard the new system as an experiment from which each and every State had a right to peaceably withdraw.” A textbook used at West Point before the Civil War, A View of the Constitution, written by Judge William Rawle, states, “The secession of a State depends on the will of the people of such a State.”
Well into the 19th century, the United States was still viewed by many as an experimental confederation from which states could secede just as they had earlier acceded to it. It took a bloody war to prove them wrong.
Fascinating Fact: It is significant that no Confederate leader was ever brought to trial for treason. A trial would have brought a verdict on the constitutional legality of secession. Federal prosecutors were satisfied with the verdict that had been decided in battle.