In Article V of the U.S. Constitution, the Framers established a simple method by which Congress, upon a 2/3s vote of both chambers, could propose an amendment for ratification by 3/4s of the states. In addition, they established a more complex method 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 or multiple amendments. Congress must verify the applications before calling for the convention by setting the date, the place and the amendment(s) authorized for discussion and proposal.

Step 2. The states must commission and send delegations to the convention where the delegates are empowered to draft and propose the authorized amendment(s) via a simple majority vote (26/50), one vote per state.

Step 3. Congress must select the mode of ratification (state legislatures or state conventions) and then transmit the amendment to the States where three quarters (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 …