Make sure the cat knows she is still important. Actually the newcomer should be sequestered and the original resident should be able to roam freely, but that is difficult when the newbie is a dog.

buy a catnip collar for the dog and bath the cat in bone marrow it should all work out fine

Cats take 2-4 weeks to adjust to changes, sometimes more. They are very territorial so when bringing in the dog she will be threatened. Just make sure she has a good high place to feel safe.

We've had our dog for about 2 years and the cat about 6 months longer. The cat still refuses to go downstairs if the dog's home and won't let the dog upstairs.
Cute dog!

Oh wow..you mentioned you were going to get a dog...maybe...so brilliant..Mabel is very cute!...is she a rescue dog from the pound?

Rub dog with catnip, and cat with a used shirt of yours. Their sense of smell will be confused and it will help.

Dogs get just one shot at messing with a cat. Once they get their nose bloody from the cat paws, they learn their lesson and stay at safe distance afterwards