In Article V of the U.S. Constitution, the framers ceded the states the power to propose and ratify constitutional amendments.
Here’s how the process works:
Two thirds of the state legislatures (34) must apply for the convention to propose an amendment and then Congress MUST call for the convention by setting the time and place.
State selected convention delegations propose the amendment and then Congress MUST select the mode of ratification (state legislatures or state ratification conventions).
Three quarters of the States (38) must ratify the amendment.
Alternately, Congress, upon a 2/3s vote in both chambers, can propose an amendment which 3/4s of the states must ratify.