Here's the good advice I received. I can't take credit, as this was my first time trying it, too!
1.) Get chicks as close to one day old as you can. They'll behave more like newly hatched chicks and have needs more similar to newly hatched chicks, which will help mom take care of them. I got one that had arrived at the farm store one day earlier, and one that had come in a shipment a few days before, so mine were probably 3 and 5 days old? But 1-3 days old is best if you can get them.
2.) Slip them under her in the dark, shortly after sundown. This will give them all night to get used to each other and she'll be unlikely to move in the dark. Remove dead/fake eggs at the same time you put the chicks in.
3.) Get them a little cold first. Leaving the chicks outside for 10-15 minutes won't kill them, but it will make them peep loudly. Let mom listen to them peeping in the dark for a couple minutes before you slip them underneath her. This helps her accept them, but MORE importantly, it makes chicks more likely to accept her. If they're cold, they're going to snuggle right up under mom and stay put, rather than try to run away or jump out of the nest. Often if there is a problem, it's not really the broody rejecting the chicks, but the chicks rejecting the broody.
4.) Check back in 15 minutes to make sure no one has fallen out of the nest, then go to bed and leave them alone until the next morning!
5.) If the broody is in a nest box, you'll need to move her down the next day so chicks can access food and water. (Real newborn chicks don't need food or water until day 2-3, so broody won't know this.) I moved mine around 11 am the next day. I set up food and water in a safe enclosed area away from the rest of the flock, and made a new nest on the floor in a sideways milk crate. Then I took the chicks out from under mom and put them down by the food. They started eating, and she followed them, clucking and complaining, and got them back under her to warm up right away. I kept them shut in their small space for another day before leaving the door open for her to take them out, and they've been doing fine since!
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