Each state legislature, most likely, will choose the delegates ("commissioners") or their state. Ostensibly, they could choose to make the selection via a referendum with the voters, but because of the time and cost involved, it will make more sense to just select the delegates themselves.
Convention of States Project, an organization I support and belong to, has a FAQ page that addresses some of the questions about how an Article V convention will be run.
COSP FAQsI recommend "What is a Convention of States?" "How Do States Choose Their Delegates?" "What Happens at a Convention of States?" "Article V says Congress 'calls' the convention. Does this mean they control the Convention and choose the delegates?" "How Do We Know How a Convention of States Will Work?" Among some of the answers in there, you'll find links to a handbook and an essay by Prof. Rob Natelson - who has done extensive research into the history of interstate conventions - as well as COSP's own handbook, which also goes into greater detail how they believe the convention will work.
Keep in mind that COSP is definitely a right-wing organization, in terms of its leadership and most of its supporters. Many of the ideas for amendments they are coming up with will not be the slightest bit appealing to left-wingers, and so will not be ratified. I first discovered COSP on Facebook.
https://uselectionatlas.org/FORUM/index.php?topic=264160.0
