The basic steps are:

1. Register a domain of your choice
2. Select who you are setting up your email with (plenty of different providers, Zoho has a basic free plan that would be suitable for a single domain and only a few users at most; Google, outlook etc. also sell services for custom domains)
3. Configure the DNS records for your domain to whatever your chosen email provider says (MX records to point to the mail server and some records for DMARC & DKIM to prevent your email being spoofed)
4. Test it all works and start using it

I’m not going to write a full tutorial so if it sounds interesting I suggest you do more research. The email hosting is typically focused at businesses as they are most likely to be wanting to host email on a custom domain.

It’s available in the Izzy on droid repo, you’ll have to add the repo to the f-droid app with the following URL:

https://apt.izzysoft.de/fdroid/repo?fingerprint=3BF0D6ABFEAE2F401707B6D966BE743BF0EEE49C2561B9BA39073711F628937A

Or scan the QR code here: https://apt.izzysoft.de/fdroid/