Associate an opportunity to one or more leads

My CRMOD application is highly customized for an 'off-label' purpose,  and the relation of opportunities to leads can be likened to options and packages when buying a car, where there are numerous options (opportunities) and they can be combined in different ways to create packages (leads).  So, many to many relations each way.  My understanding of how leads and opportunities work together in a normal CRMOD application is very poor. 

I am displaying Leads as a Related Information section on my Opportunities record, but there is no Add button, and I currently have no associations between Leads and Opportunities.  What I need to understand is how I can create associations between opportunities and leads so that the leads that an opportunity is associated with are displayed in the 'Leads' related information section on the opportunity record.