Don't bring the new dog right into the house, have a friend or someone meet you with your current dog, down the block or in a park.
Don't put the dogs face to face, just walk them along like you met a friend…let the dogs sniff pee/poo and each other butts when they seem interested.
Wait until the dogs seem to want to interact with each other and watch them.
Then walk them back to the house, and put them in the fenced back yard. If you have seen no aggression or distress, let them off leash in the backyard and let them run, interact. Then move to the house.
Remember, food, toys, hallways, doorways can trigger issues. So be aware.