Introduction with a 2-4 year old to cats.

Good morning all, my mother has dedicated to getting a rescue shepard however we have one cat which is mine and my wifes, and are possibly moving another cat in by the end of the year due to a family memebers health declining and needing to take care of them. My wife and I are house hunting so we dont plan on staying here long term however the reality is another cat will be moving in aswell.

All I've read about GSD's seem to talk about a prey drive and early introduction, however the dog thats being rescued is in the 2-4 year old bracket. This dog apparently does verry well with other dogs so im hoping animals in general it will be good with.

My big question is, what are the best ways to introduce from those who have introduced a older dog to a cat before?