If you build it, they will come. (i.e. just keep posting)
Lemmy is still small enough that even the most obscure communities will show up on the All feed, which it seems is how a lot of people browse. If you post to your small community, especially during non peak hours, some people will see it.
Well, there are a few communities automatically listing new communities and growing communities. Some people have an eye on them, or scroll through the All feed. They might notice on their own. Example: !trendingcommunities@feddit.nl
But you need some good content in your community for people to want to subscribe to you. If it looks empty or has maybe one post... It won't attract people.
So foster your community and add some content. Do that regularly for some time.