If you mean how to set https at all, you'd need the https certificate and key, place them with the correct name on the correct place (see src/be/readme) and enable ssl on global settings.
The downside of that is that most people use LE, and it requires renewel every now and then, and there is no integration. However, one could use their software and tell it to restart lynxchan after a renewal.
If you mean the redirect to force ssl usage, that's the same setting that enables ssl, it has 3 modes: disabled, enabled and mandatory.
Another option is to get a webserver handle ssl between the user and the engine running on some other port than 80.