I would wait until she has been incubating for at least 18 day before attempting introduction and use the youngest chicks you can get. When conditions ideal I can make introduction at any time of day, but if things questionable, then chicks are introduced as lights go out care taken to watch for hen pecking chicks the following morning. If hen pecks chicks, then pen them close to her for day where she cannot peck them but so she and chicks can hear each other clearly. This facilitates the imprinting process. Then repeat the placing chicks under her in the evening for the night to see of the imprinting process is furthered. There is not real magic to how the chicks and broody bond, you just got to get them to synchronize without somebody getting hurt.