The minimum is:
Table1::__pk_Table1ID = Join::_fk_Table1ID
Table2::__pk_Table2ID = Join::_fk_Table2ID
On a layout based on Table 1, you place a portal to Join with Join::_Fk_Table2ID formatted with a value list of ID's from __pk_Table2ID.
If you enable "Allow creation of records via this relationship" for Join in the Table1 to Join relationship, you can select an Id in _fk_Table2ID and this creates a record linked to the current record on your table 1 layout and the value entered from the value list links it to a selected record in Table 2.
Here's a demo file you may find useful: http://www.4shared.com/file/dZ0bjclw/ManyToManywDemoWExtras.html
Thank you, thank you, Phil. It works! I appreciate your taking the time to "spell-it-out" for me.