I have a new dog and an older cat who don't get along. Help?

I have a new dog which I got about a week ago. We watch him constantly when hes not in his crate and make sure the cats not around him. The dog is a small Chihuahua and I need some help on how to get them two more situated without being worried about them being together.

UPDATE: Thank you all for your suggestions my cat is now acting normal and isn't even bothered by the dog.