Determine the focus and point of the community. Write the description according to that so users know what to post.
Set up rules that go along with the focus. If your community is about Pikachu and someone posts cat pictures, that's something that should probably be removed.
Post some content but not too much. Don't want to make this your personal blog. You want others to participate.
Advertise it appropriately. Post to new communities. When there is another post and comments discussing your topic, mention that you have a community dedicated to it.
Getting a new community up and running is actually pretty difficult. Many of mine on reddit failed. But a few of them took off.
I'll make a note about #3. "Some content, but not too much" is probably still more content than you think it is. Lemmy has better discoverability than Reddit since it's still young, so you can probably still seed communities with just a post or two per day, but it could take you a week or two weeks of daily posts before you hook another contributor. Maybe even more if the topic is particularly niche.
At the very least, you need to seed enough content that the community does look like it produce stuff occasionally and it's just a dead subscription.