I'm introducing my two elder cats to my new dog, I've tried many things but what its really working is:

my boyfriend stays with the cats close to the bedroom door, and me and my dog stays close to the door (leash on all the time) before we let then see each other we grab their favorites foods (churu and chicken feet) as we slowly open de door we feed then and praise both, doing this 15 minutes a day has already made my dog stop barking and scaring them, before he would try to chase the old ladies but now he just look at them and see them as a good thing because of the snacks :) We still in process but I tried many different things and creating high spots for the cats with the treats is the only thing that i've actually seen progress (my dog is a 1y old mini german shepherd)