The big news here is that the instance list is automatically now, based on a crawl starting from lemmy.ml. So if your instance federates with lemmy.ml, or federates with an instance that federates with lemmy.ml (any distance), it will be included in the list automatically.

So no more editing HTML, just federate and wait for joinlemmy to be updated (we do that manually at irregular intervals).

Edit: You can find the crawler here, and maybe give it a try.