there are a couple posts across lemmy describing how to use mastodon with the service; but I have yet to find one that sums everything up neatly. also I'm doing this in my own community for moderation purposes.

1. Viewing users and communities

Any lemmy user will show up in mastodon as a normal user and vice versa, posts/toots probably won't federate properly, and toots from mastodon will not show up in lemmy for the time being as lemmy does support self-posts.

Likewise any community will show up in mastodon as a user, but this user will function like a mastodon group, you can follow the account to "join" the community and it's posts should show up in your timeline.

2. Posting to lemmy on mastodon

This is the tricky part, while it is simple in theory; due to how both platforms differ you need to keep a couple things in mind.

1. mention the account address in your toot ex: @[email protected]
2. the first line of your toot is the post title, titles do not support anything beyond plain text so don't put mentions, links, ... here
3. you can use markdown formatting on lemmy
4. the first image attached to your toot will be the preview image on lemmy, any other images will not show up at all.

3. Comments/replies

Comments from Lemmy don't seem to federate through to mastodon (I tried a whole bunch of things across various clients and I didn't manage to get a single comment through. Replies from mastodon however seem to show up in lemmy just fine.

4. Notes

A lot of this is from personal testing I conducted using this instance, I highly suggest you try some experiments yourself from another lemmy instance if you can (of course without cluttering somebody else's community without permission) and try and help lemmy and mastodon out how to federate with each other if you can.