An example:

Say you wanted to add “ShowerThoughts” from lemmy.world.

1. Go to https://lemmy.world/c/showerthoughts
2. In the right column, Look for the blurb that says something like “You are not logged in. However you can subscribe from another Fediverse account, for example Lemmy or Mastodon. To do this, paste the following into the search field of your instance: !showerthoughts@lemmy.world”
3. At the bottom of that blurb, you see where it says “!showerthoughts@lemmy.world”? You copy that.
4. Then you go to the search page on this site(from the search button in the upper right of this page), and you search for what you just copied. You may need to search for it 2-3 times for it to come up.
5. One of the results will be that community. Click the link, and then click join in the right column.

From that point forward, you should see posts from that community in your feed on this site.