First you make sure you need a union. Determine if management will discuss issues with an informal group and be willing to negotiate. Next, if that does not work, talk to your co-workers about their interest. If there is enough interest contact the U.S. Department of Labor for the proper steps.
I grew up in a union house. My father went on strike once to an amazing benefit--health coverage. The union won. There were talk of other strikes, but cooler heads prevailed. The strike should always be the absolute last resort and should not be used as a threat. A union should first serve as a means of labor and management to come together to discuss mutual concerns. Unions can present their desires and concerns. Management can explain their economic situation. It an ideal world, level headed people can come to reasonable solutions.