Everyone does this differently. I used to go to elaborate ways to keep them separate and really it turned out to be only extra unappreciated work for me.
Granted I have space, and I frequently hatch a few. And I do not count my chicks until they hatch.
What you need to consider is once they dry off, they are strong. Strong enough to be mailed somewhere. Until they dry off, they are under the hen.
Their movement under the hen flips the broody hormone from passive sitting into high alert. The older the chicks become, the weaker this hormone is. A lot of people separate them for their protection but really you don’t need to. People get into a wreck, keeping them separate from the flock, and then trying to put them in as strangers into the flock.
Anymore, all I do is put fresh bedding in the coop day 18-19. Everyone says 21 days, but often mine hatch a bit before that. The fresh bedding will allow her to leave the nest and make a clean nest.
Leaving her alone to do it her way actually works pretty good.