Describe each table and how it is related to the others. One simple way to do that is like this:
Customer Orders::OrderID = CustomerProducts::OrderID
Where the text to the left of the :: is a table occurrence name ( the names of the boxes in Manage | Database | relationships ) and the names to the right are the field names.
It sounds like you have a relationship based on a product name or description field instead of a serial number ID field but I would need to know the details first to be sure.