Introducing dogs to each other, for any reason, should be done carefully to prevent potential fights or other issues. Depending on the situation, introductions should be done slowly and always on neutral territory such as a park or another residence. Dogs should never be introduced on one dog's territory to avoid conflict.
Place leashes on both dogs so you're in control. Enlist someone to hold the other dog a safe distance away. Take both dogs for a walk, allowing them to walk next to each other but with at least a foot of space in between. If either dog takes a potty break during the walk, allow the other dog to sniff the results if he desires. Only allow this after the dog is finished and has walked away.
Once they have walked for awhile and seem calm and well-exercised, allow them to move closer to each other. Try walking them in opposite directions and giving them the opportunity to sniff the scent left by the other dog as they walk by. Allow them to approach each other if they desire, but watch their body language carefully. If either dog seems stiff, tense or aggressive, walk them away from each other. Initially, it's best not to allow them to greet each other face to face, as this can cause one or both dogs to feel threatened and act aggressively.
Taking Your New Dog Home
Once both dogs are more comfortable with each other when walking, you can transport them to their home. Keep them separated if you are putting them in a vehicle for transport. Before walking both of them into the home, put away all of the existing dogs toys, treats and food bowls so your original dog does not feel the need to protect them.
Walk both dogs around the yard of your home several times. This will allow the new dog to check out his new surroundings, and your existing dog to adjust to a new dog being on his home turf. When you bring them indoors, keep them both leashed but allow them to interact. This will allow the dogs freedom while giving you a safe way to pull them apart if a scuffle should occur. When they seem more comfortable with each other, remove the leashes.
Caution is required after the initial introduction. Only allow toys and treats when the dogs are either crated or in separate rooms. Feed the dogs in different rooms as well, and always remove food bowls as soon as they are done eating. It is vital to ensure the existing dog receives plenty of attention. Since your new dog is likely to be showered with attention, your other dog may become jealous if equal attention is not paid to him.
There are times when a little more work is required for a successful introduction. Puppies are, by nature, hyper and playful. Many adult dogs do not tolerate that well. Before introducing a puppy to an adult dog, it is necessary to exercise the pup to wear her out. Exercise and play for at least an hour before making introductions so the puppy will be more calm.
Introducing two male dogs, or a dominant dog to a less dominant dog, requires a slightly different approach. Walk the dogs as usual, but rather than side by side, the dominant dog should walk several feet behind the more submissive dog. This give the dominant dog ample opportunity to sniff the other dog. Keep moving closer to the dog in front until they seem comfortable with the sniffing, then allow them to walk side by side. Most importantly, allow the person the dominant dog is most bonded with to walk the submissive dog. This shows the dominant dog that the new dog is being controlled by his person, therefore he does not need to step in and attempt to control him.