There is no ‘definitive guide’ since there are many decisions to be made that depend on your application’s exact circumstances.
However, I wrote a guide detailing the steps I use myself a while back, which also uses a VPS on DigitalOcean or a similar host as starting point. It’s not perfect, but it might give you some initial ides: Using LetsEncrypt with Phoenix (behind Nginx). How to make this more optimal?
