Madison followed the procedure outlined in Article V that says amendments can originate from one of two sources. First, they can be proposed by Congress. Then, they must be approved by a two-thirds majority in both the House and the Senate before being sent to the states for potential ratification. States have two ways to ratify or defeat a proposed amendment. First, if three-quarters of state legislatures vote to approve an amendment, it becomes part of the Constitution. Second, if three-quarters of state-ratifying conventions support the amendment, it is ratified. A second method of proposal of an amendment allows for the petitioning of Congress by the states: Upon receiving such petitions from two-thirds of the states, Congress must call a convention for the purpose of proposing amendments, which would then be forwarded to the states for ratification by the required three-quarters. All the current constitutional amendments were created using the first method of proposal (via Congress).
Amendment process, rephrasing this in my own words and citing this chapter as a source will probably be the best thing to do