I notice that there are lots of different suggestions in the several threads on this topic dealing with many variables. This worked for me but is hardly universal advice. We have on 4yo SL Red that is a pet and we decided to expand with 8 chicks (2 each SLRed/ SL White/Black Orpingtons/Rhode Island Reds) that we kept for a while in our house. I took Rosie (the resident hen) in to visit and view her new sisters several times and at about 6 weeks moved the chicks into a small enclosed run within our fenced chicken yard so they could see each other but not interact physically. Finally we just let them all out together but with separate food and water areas available. A few times I let them all out on the lawn together. I made sure that Rosie continues to receive the petting, treats and etc to which she obviously feels she so richly deserves but if I noticed she was harassing the chicks I stopped her. On the other hand she could be the boss as long as there was no harm attempted or done. For the most part they avoided each other, By now, two weeks later, they generally eat separately but as the chicks grow they are beginning to challenge her a little, just cautiously..
I tried to make this a brief as I could without too many details but if you have questions just ask. I would call the basic approach a gradual introduction and increasing times of contact.