How can I get a dog and a cat to get along?

There is NO guarantee.  Even if the dog has lived with a cat or cats BEFORE - it does NOT mean the dog will accept a new cat, as it did the others it may have grown UP with.  I do not suggest you do this.  If you do proceed, a long slow gradual introduction with the dog on leash or muzzled for a  number of days - may be required & cat should have escape routes (like a small cat door the dog cannot go through) to a SAFE area.

It's simple.  You don't.  Why would you put your cat in the position of having to defend his territory?

I had a mastiff, Tank years ago that loved a cat that took up at our house. She would; what I call, make biscuits on him. He loved it. He didn’t want us to know. I would catch him outside and take pictures through the window. He would love on her too. He had a reputation to live up to😂 I am a dog person but I have had cats too. I have rescued kittens in boxes on side of the road and find families for them. I kept one of them when I had Tank. Tank got along with the cat a year until the cat almost killed my special needs 2.5 lb Maltese. Tank was young. We had to give the cat to my mother-n-law to save it after that. Lucky the cat came back after 7 years later, since my mother-n-law passed and Tank was fine with him. They lived together with no incidents until Tank passed from old age and a few years later Lucky passed @16. They both mellowed with age. And both lived beyound life expectancy. Tank wanted to pass alone and literally brought himself from brink of death because I was loving on him and crying. He went downstairs and passed but not alone I found him and talked to him instead of crying and told him it was okay. Lucky passed a few years later and stayed glued to me when he was getting close. I chatted with him about the snake and rabbits he brought in the house so proudly as he passed in my arms. They will always be in my heart especially my Maltese with special needs who passed earlier. My dogs lived with me first so I think if the cat is there first the dog will accept it. Rescued young dogs are usually the best dogs as they know what hard life is. They also know when they have it good. 

i don't believe in mixing dogs and cats unless brought up together from a young age. There is a good chance the cat will spend his days trying to escape from the dog. 

The best thing to do is just not get the dog. Better safe than sorry.