Introduce them with the leash on so if the cat get scared and run the dog can’t chase. If the dog is trained a couple of times not letting him chase should be enough for him to get the message. After,just make sure to let some rooms with the door open enough that just the cat can enter and have high places in every room so the can share the space and get used to eache other smells and being safe and comfortable at the same time. I have 2 🐱 and a 🐶 and the cats are the ones ruling the house.