Oh what a nightmare! Everyone makes it sound so simple and I have found that it takes longer than most told me. And it was a lot of work! I will never do this again! I did this a few years back. expanded my flock, weeks and weeks of side by side runs so they could see each other. Separate housing. It was a pain in the bum. Then I split the main run and made a separate pop door and made the new ones a space inside the coop. A couple weeks of that. Then when the chicks were as big as my other girls I went in the coop at night and put the new ones on the roost with older ones and removed all barriers. Yes the new ones still had to go through the pecking order but they were big enough to handle themselves.
I just decided to add some more to my flock, because I had a hen go broody, so I tried to put the babies under her and she rejected them and injured 3 of them. They are in the brooder box now and going to my daughters as soon as she moves. That is how much fun I had integrating flocks. I won't do it again!
But GOOD LUCK!