How can my users add a custom domain?

Hi all, I hope everyone is doing okay!

I'm building a mini SaaS, with each user (or tenant) living on a subdomain e.g: https://teena.myawepsomeapp.com

I want my users (pro users 😅) to be able to add a custom domain.

What are the steps/processes involved, from the users perpective, and from my server's configuration perpective? I'd love examples or links, or any directions... 🙏🏿

(the app will be hosted on Digital Ocean or Vultr or AWS)

Thanks.