Final answer:
The most common method for amending the U.S. Constitution, used for all but one amendment, is through Congress proposing the amendment and state legislatures ratifying it.
Step-by-step explanation:
The method of amending the Constitution that has been used for all but one of its formal amendments is Congress proposes/State legislatures ratify. This process involves both chambers of Congress agreeing to propose an amendment with a two-thirds supermajority vote, followed by the ratification of the amendment by three-fourths of the state legislatures. The only exception has been the Twenty-First Amendment, which was ratified through state conventions.