Article Five of the United States Constitution
As of 2020, the second way has never been used.  However, it exists to protect the rights of the state legislatures.  If two-thirds of the state legislatures ask them to, Congress has to call a Constitutional Convention.  Delegates would come to the Convention from each state.  The Constitutional Convention could propose as many amendments as they wanted to.  All of the amendments they proposed would move to Step 2 in the process.