How do I put interest on an invoice

Start by creating a non-inventory service charge for interest charged and then add it as a new invoice (the following "official" links say edit an existing invoice but that is rarely the correct thing to do)

https://quickbooks.intuit.com/community/Income-and-expenses/How-to-enter-late-fees-or-finance-and-service-charges-on/m-p/186347

Note in the following that automatic application of ate fees and service charges is coming to an online update in your eventual future 

https://quickbooks.intuit.com/community/Account-management/Setting-up-and-applying-automatic-late-fees/m-p/185714