Step 1. Get your own domain. I bought from porkbun. This was a difficult choice for me, having heard /read cases of domain hijacking by unethical domain sellers. Porkbun had decent reviews. Step2. Follow instructions on protonmail custom domains. Wait for all the greyed out tags to turn green. I was impatient and my domain was marked as spam by Spamhaus. Had to open ticket with them to get off their list step3: enjoy!
additional steps. I moved my dns provider to cloudflare if you wish to go that route.
TIL - there is something called Gopher and Gemini. Looks interesting, will read more on it.