How does a constitutional convention work?

To become part of the Constitution, an amendment must be ratified by either—as determined by Congress—the legislatures of three-fourths (presently 38) of the states or State ratifying conventions in three-fourths of the states. Thirty-three amendments to the United States Constitution have been approved by Congress and sent to the states for ratification. Twenty-seven of these amendments have been ratified and are now part of the Constitution. As of 2017, the convention process has never been used for proposing constitutional amendments.

February 1848 is when it was added. if that's what you were asking