Your customer's request is not uncommon. They're looking for what process engineers refer to as the PFD (process flow diagram). It's a top-level engineering process diagram.
Before handing over anything like that, you should be sure you have a solid Non-Disclosure Agreement (NDA) with the customer.
If you have absolutely no knowledge of how to diagram a PFD, then you should hire a consulting engineering firm to do it for you. It'll be well worth it to add the document to your HACCP portfolio.
However, if you do want to do it yourself, you can find many examples online. If you purchase a copy of MS Visio, it has all of the symbols and layouts for doing PFD's and it's somewhat intuitive.
This web page has just about everything you need to know...