This was my strategy for introducing my kitties:
Go very slowly with the introduction process. Rub the new kitty in a towel or blanket, and leave it for the resident kitty to smell. Do the same for the newcomer. If that doesn't provoke more growling, try brushing both kitties. Keep them separated for several days. When the growling dies down, try confining your resident cat and letting the new one roam a little. Then put the new kitty back and let the resident roam. When they get comfortable with that, let the new one roam and put the resident in the new kitty's room. With mine, I also let one cat roam, while I held the other cat so that the cat in my arms could see the cat on the ground. Later, I switched cats. That way, both were aware of the other's presence. Eventually, you can have a face-to-face meeting
You will definitely want to take this process slowly. Listen to your resident (and the newcomer) cat's responses at each step. That will help you determine when to go to the next step. It may take several days to several weeks, depending on how the cats react. If you have any major hissing/growling incidents, step back and try again.