It is natural to feel worried or anxious when your Shih Tzu is giving birth. Stay calm, and watch out for your dog. When she gives birth, you will see that the puppies are so small that they actually look like hamsters.
They are covered in blood, but do not be nervous or shocked, do not shout or scream. You are going to see that your dog seems to know what to do. She is going to lick her puppy, let her. But then observe how much licking is going on.
There are reports that some dogs do not take good care of their puppies, so when you think there is too much licking, gently get your hands on the puppy (put on a pair of gloves or use a towel, and put them inside the shoebox you have prepared).