" The procedure of secession was to have an election for delegates to a state convention, to meet in convention, and to adopt ordinances of secession. 

This was done in accord with the Southern understanding of what would be in keeping with the United States Constitution. It had, after all, been ratified by the states acting through conventions. Could they not "un-ratify" it – secede from the Union – in the same fashion? – Clarence Carson, A Basic History Of The United States