How to Process a refund from an existing service request that has been invoiced?

 I need to add a line to a service request to generate a credit memo for a service that was overcharged.  Example: The original service request was submitted and billed with a charge of 100.00

Invoices have been generated.

Now I need to create a credit memo.

Please list the steps required to create a line on the request and generate a credit memo.

Thanks Very much for your assistance.