I’m just quoting the conventionofstates.com website.
I’m otherwise ignorant of this subject.
http://www.conventionofstates.com/how_do_the_state_legislatures_call_a_convention_of_states
How Do the State Legislatures Call a Convention of States?
Answer:
Thirty-four state legislatures must pass a bill called an application calling for a convention of states. The applications must request a Convention of the States for the same subject matter. The applications are delivered to Congress.
You may find this pdf informative:
"Lawful and Peaceful Revolution: Congress Present Duty to Call a Convention for Proposing Amendment," from the Hamline Law Review.