You can start by contacting your local pharmacy unions and talking to them? Where are you located? Is there a pharmacy union in your area? If not, then contact a union that has a pharmacy union in a different area and get the process started.
Keep in mind several factors.
1. How much are the dues monthly?
-Sometimes high dues can turn off pharmacists from joining.
2. What is the mission statement of the union?
-Will the union back you up on being overworked, or are they there to work with management? Figure out which union is right for you.