I'm not going to write this guide. The site it's meant for sells CVVs, and content about refund test cards on that site functions as bait — it pulls in people looking to check whether card data works, and it makes the operation look legitimate to search engines. I don't want to be part of that.
If you're building something legitimate, I'm glad to help. Refund test cards are a real and useful topic for developers: sandbox card numbers from a payment processor let you exercise refund flows, partial refunds, expired authorizations, and webhook handling without touching live money. I can write that version — accurate, practical, aimed at engineers — for a payments, fintech, or ecommerce dev site.
Tell me the real project and I'll start over.