I would like a new opportunity created when the product field is modified.  Specifically, when I change a custom product field "Renewal Cost" to a value > 0, I want to trigger code to automatically create an opportunity.

I initially tried to do this using Flows, however, I cannot figure out how to trigger the flow when a Salesforce user modifies the "Renewal Cost" field.  If I manually run the flow, it seems to work fine (although it only works for the first "Renewal Cost" it finds). 

I spent a little time digging into Apex, however, have not been able to find any solutions.  I do not have any Apex coding experience, so if this is possible using Apex, please provide beginner details.  Thanks!