You can do it essentially as soon the quarantine period is over.
If you think both are friendly, you dont need to fuss with long introductions. (essentially, let nowcomer search out the meeting room so she feels reasonably safe there, make sure there are escape places in the meeting room, let them meet).
If you think they may have issues, take the introduction slowly and more scientifically.
Is the new a stray? You dont need to foster it? Look above.
If it is a shy semiferale, there are two ways.
Many experienced fosterers prefer to foster the new be self, "between the two pairs of eyes"..
Other take help from their resident as soon they can.
But for an inexperienced rescuer and fosterer it is easier to get help from their resident.
Sally has been here for almost three weeks but is still very shy. I did take Flaco down today, briefly, and they pretty much hissed at each other so I brought him back up where he immediately went to sleep. He has roamed around her open area, so he knows she is there. She is now up on her perch where, I suspect, he could look up and they could "talk."
I just don't want to traumatize her anymore than necessary, if you know what I mean.
HI!!! Saw Sally's picture earlier, she is just adorable :love: Here is a TCS article written by an very experienced long-time member on introducing cat's, might be of some help here. http://www.thecatsite.com/a/introducing-cats-to-cats Just take is very slowly and always have a safe room to put Sally in if thing's are moving too fast. Much luck Keep us posted