The formal amendment process that has been used the most often to reform the Constitution is the Congressional Method:
1) Two thirds of the present members of the House of Representatives and the Senate proposse and amendment.
2) The amendment has to be ratified by three-fourths of the Legislatures of the States.
There are also less used methods both for propossing an amendment and for ratifying it: The Convention Method
In this method, the amendment is propossed by two thirds of the members of a National Convention which is formed by order of the Congress after the petition of three-fourths of the legislatures of the states.
The ratification can be achieved also by the approval of three-fourths of the ratifying conventions in each state.