This has to be a two way street, not only does she have to take the chicks, but the chicks have to take her.
I have done this numerous times and have very good luck with it. You need to do it in the dark. About an hour after dark, things are pretty calm in the chicken house then. You need to make sure the chicks have a drink of water. Then about half an hour before you are slipping them, take them off heat. It won't kill them. You want them a bit chilled and peeping madly.
A cold chick should be peeping a lot, a distressing sound, the broody hen needs to hear this, and the chick needs to be uncomfortably cold. Then you stick that peeping chick into that warm place under the mama, and chick is going to burrow in and stick like a tick to this very good place. It will almost immediately get silent. The chicks have gone to sleep. The broody hen will cluck to them. They will associate that sound, smell and warmth as a good place and follow it. By morning, the chicks will smell like her.
That burrowing movement, the peeping sound is what flips off the hormones to brood into active mother hood.
Often times, with the best intentions, people drop active warm chicks in front of a broody hen, who then pecks them, those are not my chicks. They blame the broody hen when really if the chick had suddenly appeared from underneath her...well when she hatches, that is what happens.
Mrs K