Either by vote of State legislatures directly, or by State Conventions, whose delegates are appointed by and answerable to their respective State legislature.
A state legislature may determine how a state ratifying convention is chosen, but the last time around (1933), the state legislatures had the memberships of their state ratifying conventions chosen by the people in special elections.
Oh my, even better news!!!
I was wondering about that point, which wonderment boils down to: What is the practical difference between ratification by a legislature, or by a "creature" of a legislature, that is answerable to its creator?
Delegates "chosen by the people in special elections" would obviate this difficulty.
Thank you so very much, Publius!