First I'll give a warning. Baby chicks like to crawl up under the hens wings or feathers. If you handle the hen be very careful before you apply pressure. I once killed a chick by picking up a broody hen that had brought her chicks off the nest to see how many she had and what color they were. I still pick a broody hen up when I have what i consider a real need but I no longer do it out of curiosity.
We do this different ways. I typically let a broody hen raise her chicks with the flock so I let her bring them off the nest when she wants to and leave them alone. But occasionally when the coop is getting pretty crowded I'll wait until she moves them off the nest and then lock them in a shelter inside electric netting for a couple of days so she takes them back to that shelter at night where I can lock them up from predators. After a couple of days of imprinting on that shelter I open the door and let her raise them with the flock.
Don't rely too much on the calendar, that 21 day thing is just a guideline. I've had broody hens hatch two full days early, others I trust say they have seen them two full days later or more. Typically you will hear them peeping. Some stick their heads out or crawl around, I've seen them climb up on their mother's back.
I've had broody hen hatches over within 24 hours of the first one hatching and the hen already has the chicks on the coop floor. I've had some broody hatches drag out well into the third day. After the chick internal pip they start talking to Momma, telling her they are on the way. I figure the hen knows a lot more about when the hatch is over than I do so I leave those decisions to her. Others do it differently so I'll let them tell you how they do it.
I don't know how elevated your nest is. I've seen a hen get her chicks out of a 10' high hay loft, she said jump and they did, then they bounced up and ran to her. My hens typically hatch in nest about 3 feet off the coop floor, they have never had a problem getting their chicks down.