A Continental
Congress ratified the Articles of Confederation. Problems arose and they decided to call a Convention to address them.
Congress asked the states to send representatives to this Convention and these representatives formed a Constitutional Convention. I believe the
Congress that approved the Constitution and sent it to the states to be ratified was called a Confederate
Congress (or it was called a Confederate Congress perhaps at some earlier date ...no matter). This is how it all came about.
The Constitution can be amended by a 2/3 approval of Congress OR by 2/3 of the states calling for a Convention. Either way the Amendments must be ratified by 3/4 of the states.