The advice from a previous response about walking them together is a good place to start.
Here are a few more ideas to consider;
Does the existing dog (the female Shih Tzu) show any tendency toward resource guarding? That is, do you see any aggressive or threatening tendencies or can you safely get near her when she is eating, has a toy, is in a place that she likes, or with a person she is bonded to?
Has the Shih Tzu ever been reactive to other dogs? If not, that's a good start. If so, caution and maybe a little counter-conditioning could be in order.
When you are assessing the new dog, you can check to see if its play responses are intact; does it play-bow, does it give all of the appropriate calming signals when it approaches another dog, does it understand what constitutes an appropriate greeting ritual? Watch for soft eyes, a "C" shaped approach, low, wide tail wag, maybe a paw lift or a yawn. All good.
If you see a direct approach, hard,eyes, raised tail with short wags only at the tip, weight forward, agonistic pucker, it is past time to be careful.
The dog will tell you everything you need to know with its body language.
More than anything, the dogs will respond to your emotional state and expectations. If you are calm and relaxed, they are more likely to be the same.
Five moths old is still pretty young and impressionable; remember that this dog does not have an adult brain yet.
When you bring a new dog into a home environment, especially GSDs and the like, it is best to start out by confining them closely. Do not give them the run of the entire house or yard right away. If you do, this can cause problems later that are surprisingly difficult to correct. Dogs are contextual learners. Too big of a sudden contrast is really hard on them -- they understand safety by understanding boundaries. Again, more so with a GSD. Herd breeds are genetically predisposed to act like a self propelled fence -- everything behind me is mine, everything in front of me stays away. If they don't know where the line is, anxiety can set in quickly. But also watch out for barrier aggression -- dogs that would not ordinarily bother one another in any way can become extraordinarily aggressive the moment you put a barrier between them that they can see through, like a gate or a fence. This is almost like a reflex. I've seen dogs that sleep on the same bed and eat from bowls a foot apart go nuts on each other when you crate them side by side.
A crate is good if the dog is already crate trained, if not, at least confine the dog to one room and a small part of the yard until it gets its bearings. Meanwhile, crate-train the dog using only rewards.
The new dog will be in something like shock for the first three days. Be very gentle with the dog during this time, let him get used to the new smells and sounds, and give him whatever food he is already used to if you know what that is. Gradually over the course of the first week you can expand the dog's horizons as it seems appropriate. Watch the body language, it will tell you everything.
After three weeks you will start to see the dog's personality.
In about three months it will have your routine down.
During that time you can work on relationship, bond, and trust. Just remember, when a dog encounters a new situation, they ask themselves, "Am I safe? Is this interesting? What do I get?" If you can appeal to that with rewards and positive reinforcement, you can make the transition go better.
Also recognize that this is a big change for the dog you already have. Since you've been with the Shih Tzu a year already, I would suppose she already has you figured out, and she is probably already watching you for direction. If she shows signs of stress, take notice. It's worth thinking about where each will eat and sleep so they don't bother one another while they are getting their new life sorted out.
As for the actual introduction, I'd try a neutral place, like maybe even take the Shih Tzu with you when you go to check out the GSD and see if they like one another. Don't force it, let them approach one another in their own time, watch the body language, and you will know right away.