1. Put up genuinely interesting stuff on the community, at least 1-2 times per day so it has a little flow of activity.
2. Once there are a screen's full of posts or so, post the new community on !newcommunities@lemmy.world and !communitypromo@lemmy.ca. People will see the thing and subscribe to it from everywhere (if they are interested) and it will get federated to most of the main servers.
3. As the activity continues, people may decide to post stuff of their own there, and other people will run across it either from the "local all" feed of your instance or from the "global all" feed somewhere else. They may subscribe to it, they may happen to see a cross-post from someplace they are subscribed to, but however it works out there will grow a steady flow of subscribers, if people are interested in it.

The sort of filtering aspect where not every new community will automatically get its posts thrown into every single person's feed every day, is a feature not a bug. The steps above are enough to get it popular if people are into it, but also limiting enough that it won't get publicized above its organic popularity level and start spamming everyone's feed.