When which is involved in a relationship, you will always have to populate the field such as [Related parent].
So it depends what the key field is of the parent record. Sometimes you may actually have a known customer number and you can choose to make the key field of your customer table that customer number. But another situations you might just have a Record ID being the Key field of the customer table.
If that is the case then in order to join up the orders you would need to populate that upload file with the record ID number of the customer somehow. Sometimes the only way to do that is to download the customer table into Excel and then do a Vlookup on the customer name in order to get the Record ID number on the orders table and then you do your plate.
