There's a "new communities" community that you can try posting to (I'd provide a link, but I'm still kind of figuring things out myself)
Other than that, biggest thing is probably just to keep up with it, if you build it they will come. Make sure you're keeping on top of whatever moderation tasks need to be done, no one wants to join a community that's overrun with spammy bullshit or has a crappy vibe. Create content, people also don't want to join a community where no one is posting (but don't get spammy about it yourself, personally I think it's a bit of a turn-off if I see it's all just the moderator posting himself day in and day out) engage with your members, upvoted, comment, ask questions, offer answers, be the kind of member you want to attract.
Be patient, things are growing but Lemmy doesn't have reddit numbers yet, we're all learning, things are changing fast, but building good communities will take time.
And of course, promote yourself like you're doing here (again without getting spammy) if you see people talking about bass, work it into the conversation. Lurk the music/musician subs, make the occasional shameless self promotion post.
Maybe trick a few fishermen like me into joining who thought it was about the other kind of bass.