Look up "portal" in FileMaker help. You'll most definitely need a table for the PO's and a related table for each item listed on that PO. You may also need to set up a QA notes table that you would link to either the PO and/or individual item records.
Here's a greatly simplified invoicing demo you can download and study. It was created by someone named Comment who used to be active in this forum. He uploaded it to a different FileMaker form than this one:
Purchase order systems, typically use the same basic structure, it's just that you are buying instead of selling so it should be a good place from which to start.