I don't know how you handle your eggs for eating. I'd keep gathering all eggs for eating as you are currently doing. When a hen goes broody, you need to wait a couple of days to be sure she is really serious about being broody. My test is that if she sleeps in the nest two cionsecutive nights instead of on the roosts, she is ready for some eggs. You can collect eggs for her while you are checking to see if she is serious about being broody.
When you think you may have a broody, start collecting eggs for hatching. Try to get clean eggs that are regular size for the hen. Egg size can vary by breed, but you do not want an egg that is excessively small or excessively large for that breed for incubation. The ideal is to keep them in a place around 55 to 65 degrees, but many of us don't have those places. Just do the best you can. A place with higher humidity helps too, but again, not all of us have those either. Store them in an egg carton with the pointy side down. What you are actually trying to do is keep the air sac up. You can turn the eggs a few times a day if you want to, but it really is not necessary for a few days. To turn them, get a block of wood and put it under one end of the egg carton. When you turn them, move that block of wood to the other side. The idea is that you don't want the yolk to settle against the side of the egg shell. It should not take you many days to get all the eggs you need for her.
The embryo will not start to develop until it is close to incubation temperatures, but it does start to develop when it hits incubation temperatures. It varies a bit, but you might actually see some development if you crack it about 17 hours after it starts incubation. The egg should hatch in about 3 weeks, sometimes a day or two early or a day or two late, even under a broody, but somewhere around 3 weeks.