Where is the new rat coming from? If he/she was with other rats, then quarantining is a good idea. This should be done idealy in a separate homes but many have done it successfully using a different room in the house. Doing that, you'll have to be careful when playing with the new little one and then playing with your own.
Quarantine can be from 2 to 4 weeks, longer if you feel there might be something off with the new rat.
Introductions can be simple or take a lot of patience in time. What are the ages of the new rat and the resident rats, also females or males and are any of them altered?
Would you have to do in-home quarantine?
Lets start there.
The best quarantine is 3 weeks in a completely different building (different air space), with you visiting and showering and changing clothes after handling them.
No so good, but happens is home quarantine. Different room as far as possible. Handle the residents first if possible, then handle the newbie(s). Afterwards shower, change clothes etc before going near your own rats.
This will not work for those scary contagious diseases like SDA, Sendai, etc, since they are in the same airspace and these are airborne viruses.
Watch for lice or mites, any signs of illnesses that can pop up within those 2-3 weeks. New home sneezies are very common with babies and usually subside after 2-4 days. If it continues much past that or any secondary symptoms show up, then you will need to take her to the vet.