The number one rule to adding a new dog to your pack is to do it gradually. The worst mistake that people make is to just bring the dog into the house. To the dog that is already there this is an intrusion of his/her territory. To the new dog, being thrust into an unknown environment leaves it without any rules or boundaries to respect.
To introduce your pets you will need the help of a friend or family member who is well known by your current dog. You must all meet on neutral territory. This should be a place that is not familiar to either pet.
You want to drain energy from the pack and allow them to become familiar with each other in a place that none of them “owns”. This will be done by going for a long walk.
At the start of the walk, you take the lead with your existing dog, while your friend or family member follows behind with the new dog. After awhile, drop back and let your original dog sniff the new dog, but don’t let them meet face to face yet. Continue thewalk with the original dog in front and let the new dog sniff.
Bring the entire pack together, with the dogs walking on the outside and the humans in the middle. When they are in a calm state and walking together without incident you can bring them home.
When you get home, take the original dog inside with you, then bring the new dog in. This allows your original dog to invite the new dog into their territory.
Once your dogs have become a pack, it is important that you let them establish hierarchy among themselves, with you and other humans in the house as pack leaders, of course. Don’t show favor to your original dog and try to make them the dominant one.
If you try to force a submissive dog to be dominant it will only make the submissive dog very anxious and insecure, while making the dominant dog resentful. Your new pack will let you know which dog is dominant and which is submissive(or they will take equal positions.) They will be happier for it if you let them make this rule for themselves.