All you need to do is just:

1.) Create the sublemmy and set a permament name and displayed name for it.

2.) Set a baner and icon to make it more attractive or noticeable when you’re scrolling through local feed.

3.) Write rules and info about the sublemmy on the side bar.

4.) Start regularly posting things on your sublemmy to attract users.

5.) Moderate your sublemmy by checking if all posts follow the rules and reacting to reported posts/comments on them.

I think that it’s everything. There are also a few sublemmies for sharing newly created communities which could give you a few subcribers for the start.

Edit: if you want an example of a side bar then you can check a side bar of my community with over 1250 subscribers: [email protected]