First you need a willing and capable handler to help. Next you need to be on truly neutral ground. What is "truly neutral"? It's an area that your dog doesn't regularly visit. Home won't work well, the sidewalks on your street probably won't work and the park where the dog has a favorite bush might not be the best idea either. This is a bummer for me since my dogs cover 4-8 miles a day in this area. There are very few places my dogs don't consider their normal stomping ground. We usually have to drive somewhere or use streets and grassy areas that we don't go by on our walks. Another thing to consider is the disposition of the dogs. You may need more space than a street or small park will allow if one of the dogs have had issues in the past.
Ok, you've found a nice open spot and have a willing helper. When I start, I don't want the dogs to have line of sight to each other. Ideally the dogs are on far sides of the park and ready to walk. You both need to be moving in the same direction, and the more distance you have available the better. Have both people set out walking and follow this pattern until one of the dogs react (and by react I mean showing any signs of intensifying interest like raised hair, barking, staring, growling etc)...
Once a dog reacts, you want to increase the distance between handlers until both dogs are again just walking. NO CORRECTIONS, NO VOCALIZING ANNOYANCE, NO ANYTHING, just keep moving. Now from the other end of the area, turn around and repeat. But this time start out just a couple of feet closer. You may end up pacing the park dozens of times. I take as much time as needed to get the dogs within leash distance, and with one dog aggressive dog it takes me a while to get there.
When the distance is comfortably getting close enough that the dogs could easily reach each other, place the handlers between them and continue walking parallel until the dogs have more interest in their surroundings than the other dog...
By now you've gone back and forth countless times slowly bringing the parallel paths together and both dogs are nicely walking and sniffing the ground without much care that another dog is right there. On the next pass, have one handler hand off their dog and the now lone handler should walk with the dogs on opposite sides...
If both dogs are well socialized you might luck out and have little to do from here. But I continue on between the dogs and we up the stakes by allowing the smell introductions. For this I choose a tree or bush that is close to a sidewalk corner, ensuring that it is full of wonderful smells from various other dogs' markings. Let one dog have free run of the entire leash and lead it to the bush/tree. While this dog is immersed in the smells, let the other dog move in but NOT close enough to get muzzle to muzzle. Normally, the second dog will take this opportunity to investigate the other dog in a dog like way. Don't linger too long, I find that stopping or standing still for too long increases the odds of a fight. Choose another bush or object and head out again between the dogs. Now reverse the order of who's smelling the object and who is smelling the other dog. Repeat this dance again and again until the dogs no longer go in to smell the other dog...
Now we're at what I call the final stage. After the process above is over with, I will continue walking both dogs (on whatever side they choose to be on) for at least 30 minutes. Usually this involves some running on my end. I want them to be absolutely worn out and flooded by the experience up to now. Ideally you were close enough to home that you can walk there.
While you're out walking the dogs, your helper should be back at your house and removing all toys/prized possessions from the house and yard. As you return, have the helper take the primary dog (dog that lives there) into the house and you take the secondary dog to the yard. Let that dog investigate the yard solo until it's no longer interesting. Now is when I remove myself from the equation and I let the primary dog outside and that's it. Happy dogs playing in the yard.
Some additional points: This method combines things I've read from dozens of sources. But it has been tailored for my use because I have to deal with Coco and her intense dog aggression. Time and distance are the only things I've found to get over that hurdle.
ONLY USE ONE DOG AT A TIME IF YOU HAVE MULTIPLES. Even though I have four dogs, I take the time to do this with each one. The last thing I need is a pack mentality taking hold during the introduction stage.
I have also purposely eliminated any sort of treats in this process. Mainly because two of mine are resource guarders when new dogs are present, and also because I don't know much about the new dog and its tendencies. If anyone has a treat based method for introductions that they have had success with, feel free to post it here.
