The example below creates a new "Many-to-Many Relationship"  between the Opportunity entity and the seminar entity.    The image below lists all the active seminar records in our system.   You may want to track all the seminars associated with a given Opportunity and Vice Versa.     The steps below will illustrate how to create the relationship you need.

Current Entity
     - Enter the "Custom Label" you wish to use
Other Enity
     - Select the entity name and enter the "Custom Label" you wish to use