To create parent child relationships with records already entered into custom tables
Click File > Create parent-child relations...
Set the parent table
Set the key for the parent table
Set the child table
Set the key field for the child table
Click the process button to relate the tables
The overwrite checkbox will if checked create the relation if a match if found, if unchecked it will create the relation if there is a match and the parententityname & parententittyid fields are blank.