How to calm Shih-tzu while grooming?

Do you use clippers or scissors? Try introducing her to the scissors, let her sniff them, give her a treat and put them away. If you are using clippers so may be afraid of the sound. So, again, let her sniff it and give her a treat and put it away. After a while try turning them on so she can hear the noise, and give her a treat. Be patient! She needs to see it as something positive, so if she is use to seeing them and getting a treat she'll get excited and perhaps remain calm.