If you have a mature flock, you'll want to raise your chicks in a brooder for the first 6-8 weeks. Once they are mature enough to be introduced to the coop, you will still want to keep them separated from your existing flock during an acclimation period. Put them next to each other where they can see and hear each other, for up to a week. After this time frame, you are able to integrate the two flocks. It's best to do this around dusk when they are more docile.
There will be a normal pecking order to follow after. This is the best gradual introduction you can make because if you were to place them without the introductory period there could be dire consequences or injury due to the smaller birds being cornered by larger birds.