Best place to start is the NIP: https://github.com/nostr-protocol/nips/pull/542

The code for zapgate.link is also publicly available at: https://github.com/lightning-digital-entertainment/zap-gate-demo

and I will make sure that we get the backend code live asap.

You can reach out to me or #[5]​ at any time. We are happy to help with integration.