That's the fun part.
Nobody has any idea how that would work, because it has never been done. There's a mention of it as part of the process in the Constitution, but no instructions how to do it. The several states would have to essentially manufacture a process to set up their delegation. Practically speaking, the option doesn't exist.
And yet a substantial number of states (nearly enough to trigger it, depending on how one counts it because of course that isn't clear either) have called for one!
Doing a constitutional convention on this that or the other has had waves of popularity over the decades, and most experts involved (who aren't pushing a particular constitutional amendment that's in fad) are pretty horrified at what would happen. Hypothetically states could be effectively locked out, the convention could just redo the entire constitution with no accountability once they're there, etc. etc.
From what I remember most states that are currently calling for a convention via state legislation (again depending on how it's counted, etc. etc.) have done so to force a balanced budget amendment to the federal government. Back when I did a bit of work relating to the question of constitutional conventions there was a lot of concern that it'd become red states forcing a ban on birth control or banning gun restrictions. Regardless of how it shakes out it'd be a pretty destabilizing event for the country.