The first thing to say is that it is so easy to get it wrong! Bringing a new dog into a house where one already lives has lots of potential for growling or snarling. Even worse than that they will either try to mate with each other or just get straight into fighting for their territory. After all the resident dog is king of the castle already and may not take kindly to an interloper.
It doesn’t have to be that way though and, if you are lucky, they will get on really well from the outset. Let’s not forget that besides being a territorial sort of animal most dogs are quiet sociable. One that has had a lot of time in the house alone might actually welcome the company and enjoy having a playmate. But, just for now, let’s assume that you are NOT that lucky and that you have quite a bit of work to do before the new dog is made to feel welcome by the other one.
You should remember that you need to prepare both dogs for the moment when they will be sharing the house and one way might be to make a big fuss of your existing dog BEFORE you go to collect the new one. That way you will have a fair amount of doggy smell on you and the new dog will immediately be aware of that. It might make them think “Hello, I’m not going to be an only dog”. Giving the dog lots of attention on the way home will help a lot, creating a bond as quickly as possible. When you get home it should therefore not be too much of a shock to find another dog in residence.
Assuming though that the first meeting is not too cordial it might be wise to keep the new dog in an open sided cage (if you have one) in full view of the resident dog. Let them have a good sniff at each other without the opportunity to come to blows (or bites). After a while the top dog will be so curious to get to know this visitor that he will be less inclined to pick a fight.
It’s really a question of letting them do their introductions in their own time and offering a few treats for good behaviour might not be a bad idea. Everyone in the house needs to remain calm during these first few hours, and possibly into the next couple of days. One dog should not be given special treatment at the expense of the other, and they both need to see that you are behaving fairly. In time, hopefully a very short time, they will be bounding around, playfully yapping at each other and you will have a pair of happy dogs.