Whether you are just starting out, or you've had an existing flock for awhile, everyone eventually adds new birds.
To assure that this is done as easily and safely as possible, there are a few basic steps to take.
Slow and steady really does win the race. It is so very important to not rush through quarantine, visual introductions and eventual addition of the new birds.
If you are introducing chicks that were hatched by your hen, they will protect them and you will not need to worry about the introduction to the flock. If you are introducing chicks that you have incubated yourself, you will not need to quarantine per se but you do need to wait to introduce them to the flock until they get to be closer to the same size as the rest of the flock.