There are a few things you need to know, but after that you have a lot of options.
First, it is very important all the eggs start at the same time. For your first time especially you don’t want to deal with a staggered hatch if you don’t have to. The eggs will hatch about 21 days after incubation starts. It may be 19 days, maybe 23 days, but 21 days is the target. Normally they all hatch within a couple of days of each other, whether early, late, or on time. So collect all the eggs you want her to hatch and start them at the same time. Store them in an egg carton pointy side down in a relatively cool spot in your house. Find some place that the temperature is relatively stable, not in sunlight and not under an air vent.
There are a lot of different signs a hen might be broody. The way that convinces me is that she spends two consecutive nights on the nest instead of roosting in her favorite spot. What goes on during the day does not convince me. One night on the nest does not convince me. Two consecutive nights.
Now you have a decision to make. Are you going to isolate her from the flock or are you going to allow her to hatch with the flock. There is not a right way or a wrong way in this. People do it successfully both ways and have been for thousands of years.
If you let her hatch with the flock, you need to mark each egg you give her. I use a black Sharpie and make a couple of circles around the egg, one the short way and one the long way so I can tell at a glance which eggs belong. Then every day after the other hens have laid, check under her and remove any eggs that do not belong. As long as you gather them every day like this you can still eat them. You will not see any development in them.
If you decide to isolate her there are several different ways to go about it, but you need room for a nest, feed, water, and some extra room for her to go poop. It needs to be predator proof too. The broody knows to not poop in her nest but she might poop in the water or feed. You need access to clean it out, plus you need to provide her with fresh water and food anyway. Many people lock off an area in the coop, some move her to a totally separate building, some even move her into their house. Yeah, really!
When you move her, do so at night with as little light and commotion as you can manage. Lock her in her new condo so she cannot go back to her old nest. Put some sacrificial eggs or fake eggs in her new nest. That’s the biggest risk in doing his, she might not accept the move and break from being broody. Once you are confident she has accepted her new nest, give her the eggs you want her to hatch and remove the sacrificial or fake ones.
People may add their tricks and ways to do this for you. We all have our own experiences and ways we do things. Good luck!
