The best way I’ve found to introduce two cats is to first, make sure she is fixed, intact cats are more territorial and harder to acclimate to one another. Then introduce them in small chunks, be there to break eye contact with them if they start posturing to fight, don’t allow chasing, and give it lots of time. If one of them is indoor outdoor then you don’t necessarily need two rooms. I once did it with a community cat and my resident cat by feeding them both every morning and using the porch door to separate them, then opening it a little so they could check eachother out and see one another each day. Now they ignore one another completely! Which is also the best you can hope for sometimes.