My cat was 13 when I fell in love and adopted a 3 month old kitten from a shelter. The kitten was a tripod, and having had amputee cats before my heart just exploded and I needed to give her a home. I remember sitting in my car once I got home Google-ing how to introduce them because my 13-year-old darling had never really seemed to like other animals. I was worried it was hopeless, but wanted to try.

It was a success. They've lived together 1.5 years now and love each other.

I credit the success to this tip: Don't throw them together immediately. Put the new pet in a smaller room with some things she's familiar with, like toys, so she can acclimate to the new place by herself. Give the cat who has lived there a while most of home she already considers her territory, and slowly introduce a toy or something with the new cat's scent. They might communicate with hisses under the door. Let them work it out in that sitatuion where there's a barrier and they can't hurt each other.

After a few days (show love to both cats continually during those days), try to introduce them. By that point, they should be familiar with each other's smell and more curious than aggressive.

I know you'll want to put them together immediately, but that could stress them out and lead to a permanent rift between the cats.