Quick question on implementing structured data

Hey guys, quick question on implementing structured data. It`s all pretty straight forward so far, but I`m unsure of what to do for product reviews and how to implement it correctly. I should mention I`m using json-ld, and the product reviews are done through a bespoke system built into the site (oscommerce). The customer is required to log in before leaving a review. The client already has a lot of reviews, so I thought it would be good to add structured data to them.