Here's how:

https://www.hort.purdue.edu/newcrop/morton/roselle.html#Harvesting

The flower falls before the red calyx enlarges and becomes fit for food use.

Lazy- thanks for the great website. In complete (almost) ignorance, I planted the Roselle last spring for the calyces to make tea or juice, with the intention of trying to preserve the plants through the winter. They are clearly annuals and I'm not sure if or when they would bear fruit if they survived. I think I'll try to save a few (I have about a dozen plants) just to see what happens, and let the rest go with the frost and replant in the spring.