I used fiatjaf's bridgeaddr (https://github.com/fiatjaf/bridgeaddr)
Most of it is explained there. Let me know if you run into any snags
Discussion
thank you sir, another question, are you using the domain for anything else other than receiving zaps?
Once you set a domain (or subdomain) in a CNAME record to another address, all traffic to that domain (or subdomain) will be forwarded to the address in your CNAME record.
In my case I just used a subdomain in my CNAME record.
So, I have other subdomains free to do other things
So invoice.gerbils.online is exclusively used for mapping LN addresses to my LND