Instructions
Put both dogs on leashes and have a friend, spouse or other person hold one leash.
Allow the dogs to sniff each other and check each other out. This is how dogs greet one another and shouldn't be discouraged. Make this relatively brief; if one or both dogs is feeling threatened by the other, too long of an introduction may escalate into a violent fight. Don't force this introduction, let it come naturally to the dogs.
Give each dog praise and positive reinforcement while they're in each other's company. This will help the dogs understand that it's good for them to be together. Positive reinforcement includes talking in your "happy" voice and offering small treats.
Study each dog's body postures and signals. If one dog goes into the "play bow," where their rear is in the air and their head is near the ground, this is normally a good sign. The play bow says to the other dog "Let's play!" Warning signs of a bad introduction include the hair standing up down the back, sometimes known as "hackles;" deep, threatening growls from one or both dogs; stiff legs, posture or a stiff wag of the tail; or teeth baring. If one or both dogs shows any of these warning signs, you and the other handler need to calmly call both dogs back to you and get the dogs' minds on something else like play or treats.
Allow the dogs a shorter interaction time after they have completely calmed down from the aggressive body postures. To do this, you may choose to let them interact at a distance, or just shorten the duration of the greeting.
Take the dogs for a walk together, while still with the other handler. Keep the leashes loose and make sure the dogs are both even -- not one in front of the other. Dominant dogs always lead the pack, so keeping them at the same pace and parallel can help curb aggressiveness. Keep the dogs separate on the walks, although even. You don't want them forced to be next to each other if they're not feeling it.
Keep all treats, toys and food off the floor for the first couple of weeks. These are resource items that one or both of the dogs may feel they need to protect. Dogs protect their resources through growls of dominance, and if they haven't set up their dominance hierarchy yet, these items will cause squabbles.
How to Aquaint Dogs to Each Other at Home
Introducing a new dog is not always an easy task; dogs are territorial and have their own system for determining which dog is the leader of the pack. While you can attempt to do this at home, it is always best to introduce the two dogs in a neutral location, such as a park, or any area that is unfamiliar to both dogs so that neither has deemed the area its "territory." Dogs will show you how they're feeling about each other with body postures; you'll need to learn what's friendly and what's not.