Hi, welcome to the forum. Glad you joined!
There are lots of different ways to go about this. What might work best for you will depend on different things like how much room you have, the quality of that room, how many new chicks you will have, what the weather will be like at critical times, whether you brood them in your house, in an outbuilding away from the adult chickens, or next to the adults so thy can see each other and other things. It's challenging for me to give specific suggestions when I don't know what you are working with.
One very important aspect is room. That's not a square feet per chicken thing for integration, especially when there are maturity differences involved. It's more of a case of whether or not the weak can void the strong if they need to, hence the quality of that room. That's where a safe haven/panic room can come in handy for chicks if your space is tight.
In general I find my pullets tend to avoid the mature hens until they mature enough themselves to start laying. They may get along fine in the run or coop during the day with no chasing or attacking. But they tend to form a sub-flock and avoid the older ones. At night the pullets tend to not sleep on the roosts with the adults, even if they are all mature, Mine tend to bicker as they are settling in for the night, they can be fairly brutal to younger birds. Don't be surprised if your pullets look for a different place to sleep.
I raise my chicks in a brooder in the coop. The chicks are basically raised with the flock. I have a large coop and a lot of room outside. I also have weather so my adults spend practically all day every day outside, except when they are laying. I typically open the brooder door in the morning when they are 5 weeks old and walk away. When they are all out I lock the brooder door. That's it, integration is done. That's how easy it can be if you raise them with the flock and you have a lot more room that that magic 4 square feet in the coop and 10 outside. Most of us on here don't have that kind of room. I don't recommend you try it that abruptly until you have a bit of experience.
I'll suggest some generic things. How or even which you implement will depend on your unique situation. As much as possible allow them to spend time where they can see each other but not get at each other. If you brood them in the house maybe section off a portion of the coop for them or even provide a "grow-out" coop and run either in the main run or right next to it so they can see each other. A week like that might be enough but longer does not hurt. It can be easy to convert these to a panic room/safe haven if room is tight.
When you allow them to mingle provide food and water at separate areas to avoid forcing conflict on them. By separate I mean places as far apart as you reasonable can. Some people have said that doing that did not work but their areas were only a couple of feet apart. That was not separate.
Improve the quality of whatever space you have. A lot of us do not have the luxury of real room. Giving them places to hide under, behind, or above can be a huge help, just don't create a trap. Leaning a piece of plywood against a wall or fence can work well, but secure it so it cannot fall. Or put something on cinder blocks. Some people find putting a table in the coop or run to be useful, they might even feed or water up there. The idea is to break the line of sight or give them a place to run behind of they need to. Perches high enough that the adults cannot peck their feet can be useful.
I think these are the highlights though I'm sure others have some more tricks. A lot of us do this on a regular basis without any real drama, but occasionally there are disasters. It just works that way with living animals, no one can tell you for sure what will happen. The better you can do the better your odds. Good luck!
