Help vaultwarden + nginx https without domain (local network only)

Hi all.
Situation (under ubuntu desktop):
I have deployed Vaultwarden as a docker.
As I am requested to use HTTPS, I have read that I need some kind of reverse proxy (I don’t know what that means)…so I deployed nginx as a docker too.
Both services work:
Vaultwarden under port 8088
nginx under port 80, 81 and 443
But I don’t know how to correctly setup a certificate (self signed?) that makes Vaultwarden work well.
Is there a STEP-BY-STEP guide (that works!) to do this?
I followed a lot of step-by-step guides to do this found over the internet but each of them results in one or another error which leaves me frustrated.

Note that I can’t use a domain. Can’t redirect ports and I don’t want this environment to be accessed online. Just a local network system.

I’ve spend two days (more than 14 hours) trying to deploy a working environment. Tried under Windows and also Ubuntu.
I managed to make it work under Windows but other problems (can’t autostart dockers) made this solution useless…so I need to have it work under Ubuntu.

Can somebody help?
Many thanks and apologizes because my english is not good.