Well, the “how” is technically simple. You paste the URL to the search box and you hit subscribe. You can do that right now with:
- Lemmy communities
- Kbin communities
- PeerTube channels
- Mayyyyybe a.gup.pe / chirp.social groups??? idk how well those would work
Lemmy itself only let you subscribe to ActivityPub Group actors though, so it’s quite restrictive in that regard. kbin adds user follows and microblogging into the mix, but you can’t do those through Lemmy yet (or perhaps ever).
However, the real “problem” is presentation. While you can, say, follow a Lemmy group from Mastodon. Mastodon is not intended for groups so it kinda breaks and ends up spamming your home timeline with all the posts and comments. Other implementations such as Akkoma or Misskey or Calckey (pending rename) might end up interacting better (because Mastodon will try to convert everything it gets into Notes in a “lossy” fashion).
While the protocol does allow you the freedom to interact between services, you will not get the best experience if you’re not on a “similar enough” service. Although that does not stop you from following a PixelFed account from Misskey, or a Mastodon user accidentally finding their way into the Lemmy comments section. (You can tell because they’ll be the only comments that end up tagging people when replying)
alongside the others mentioned here, lemdro.id exists for Android-related stuff and lemmyrs.org exists for Rust stuff (but seems to be running an old version of Lemmy, so YMMV)