Just some of my thoughts for you but others with more experience may have other ideas.
I recommend at least a two week quarantine period when the gals arrive, just to ensure no diseases are transmitted to your existing flock.
I have found time and patience to be the best method.
Initially, I would try and separate them with a wire divider or similar so that that can all see each other but not touch.
As space is the best tool with integration, I then let them have supervised free range time but pop them back into their divided runs at lock up time.
In the beginning, if the original gals are free ranging, I close them out of the run, remove the divider and let the newbies explore the areas they have not been in; this gives them the opportunity to become accustomed and learn where to run and hide if need be. An area they have never been in plus already settled occupants can be traumatic for newbies so they will benefit from being familiar with the surroundings.
I also switch this around and let the newbies out for a free range and keep the original gals locked in; giving the newbies the same opportunity to get used to the garden, hiding places etc without the original gals giving them grief. [Note: the original gals are never impressed when this happens and have no qualms vocalizing their disapproval ]
If you have an existing hen who is fairly easy going and placid, you could try introducing her to the newbies and if she bonds, she may be able to help introduce them to the flock. If you have any bullies, I would definitely keep a close eye on them.
Distractions are also a good thing to have around so the original girls have something better to do than chase and peck the new ones. If they are in the run together, maybe some leaves or clods of dirt for them to scratch in or hang a head of cabbage for them to peck at. If you have the room, some hiding places for the newbies or extra branches etc will be helpful.
When they are integrated in the run, I also provide two lots of food and water for a couple of months to ensure that all have access.
I carry a water pistol to break up any squabbles that get out of hand. A spray of water can help them to change their mind about going in for a peck and my gals have learnt that if I get the water pistol they are doing something naughty Note: it does not hurt them and after a few sprays, they get the idea.
There are some good tips on this BYC thread also:
Good luck with them and please let us know how you go?