It's really going to depend on what you're trying to do.

How did you set up your lemmy instance? Did you use the ansible script?

Which part in particular are you having trouble with?

One puzzle piece you may be missing is that you're going to have two different websites (a photon one and a lemmy-ui one) and they will (I'm assuming) both be on the same server. You'll need a way to direct traffic to the right place, and that way is a reverse proxy. You might reuse the nginx that Lemmy uses, but I find it's cleaner to leave Lemmy to do it's thing and set up another reverse proxy (which might be another nginx or it might not). Some other popular ones are nginx proxy manager, caddy, traefik, haproxy.