How can I have deduction element automatically added to entries when choosing bargaining unit

I created a new bargaining unit in the Flexfield values and also a new deduction element.  When I choose the bargaining unit I know there is a way to have the system automatically put the deduction element on the employees element entries.  I know I could choose the eligibility criteria in the element screen to limit the element to only those individuals but I am not sure what is next.  I have other unions set-up this way but have not had to create a new one in a very long time.  Can someone explain the steps to do this?

Monica