In Article V of the U.S. Constitution, the Framers established (1) a method by which Congress, upon a 2/3s vote of both chambers, could propose amendments for ratification by 3/4s of the states, and (2) a more complex process by which the states could do the same:

Step 1. Two thirds of the state legislatures (34) must apply for a convention to propose an amendment.

Step 2. The state convention delegations must draft/propose the amendment via a simple majority, one vote per state.

Step 3. Three quarters of the States (38) must ratify.

The text of Article V (convention text highlighted ):

The Congress, whenever two thirds of both Houses shall deem it necessary, shall propose Amendments to this Constitution, or, on the Application of the Legislatures of two thirds of the several States, shall call a Convention for proposing Amendments, which, in either Case, shall be valid to all Intents and Purposes, as Part of this Constitution, when ratified by the Legislatures of three fourths of the several States or by Conventions in three fourths thereof, as the one or the other Mode of Ratification may be proposed by the Congress …