I'm a moderator of a few communities here, most notably !documentaries@slrpnk.net, !buyitforlife@slrpnk.net, and !mealtimevideos@lemmy.world. Before migrating here from Reddit, I was a moderator on r/LinuxHardware, which grew to become a sizeable community of 76,000 subscribers.

Here's my takeaways on growing a community:

1. Unless the topic of the community is extremely popular from the get go, you're going to have to supply content to the community until it becomes self-sustaining. The more niche a topic is, the longer you will likely need to supply content to keep it alive. It's important to be passionate about the topic, and to have a good idea/plan of how you're going to find or create more content to fill it with for people to engage with.
2. Connect with other similar communities so people know yours exists. r/LinuxHardware was semi-popular from the beginning, but soon after its creation, similar already established communities added it to their sidebars, which made its growth explosive.
3. The quality of your engagement in the community matters, especially if it's a help related community. If you're well versed in a subject and can provide high-quality answers or suggestions, people will notice that.
4. Getting the people involved in aspects of the community is a good way of creating a sense of co-ownership. Have discussions or votes on how the community could be improved, have a friendly contest of community submitted art for the logo/banner.
5. Be stern when it comes to toxicity, as allowing that to fester can and will put people off from wanting to engage in a community. A warning is a good start, but don't be afraid to make enemies if bad behavior continues. You'll never please everyone.
6. Show appreciation for people showing up to engage in your community. Upvote their content, and leave a nice comment if you can. Doing so signals to other people that their efforts are being seen and appreciated, which encourages people to come back. Positivity tends to breed more positivity. :)

As for what not to do:

1. Don't engage with trolls.
2. Don't moderate while hungry, angry, or while having a bad day.
3. Don't assume the worst of people, unless it's abundantly clear that the worst is absolutely true.
4. Don't have an excessively long list of rules, just the things that really matter.
5. Most people will only read a small amount of the sidebar, so make things short and sweet, and keep the most important pieces of information at the top.
6. Don't burnout and post everything you can think of all at once. Consistent and steady output is better than a lump sum, as it shows the community isn't dead.

I would recommend anyone starting a new community to seed it on the Lemmy Boost project. This service makes a bot on all participating instances subscribe to your community, which will make it show up in their user's All feeds, and can dramatically help with natural discovery.

Lastly: Try to have fun! Express yourself, be quirky, add little splashes of color to your sidebar (like this cool little subscriber count button, which incidentally I first saw on Open Course Lectures, so thanks for that Spaduf). If you're not having fun, it's unlikely you'll be able to sustain your community during the critical phase before it is self-sustaining.

But that's just my 2 cents. :)