ATM, you can’t. Normal mastodon posts are not understood by lemmy servers. They don’t know how to handle content that is not associated with a community.
Most of the fediverse is like twitter. Users making posts to their own “microblogs”/profiles, following each other or browsing a timeline of all posts by everyone. That’s mastodon, and it has by far the most activity.
Lemmy doesn’t support profile posts, and you can’t follow users, only communities.
Basically, all content on Lemmy is posted to groups, while all content on Mastodon is posted to the users own profiles. While the networks are technically connected, the content type is not compatible.
I hear mastodon is getting support for groups, though, which might be something that can be interoperable with lemmy communities. Then they could look at communities as if they were user groups, and post to them, and we could sub to mastodon user groups, and see their posts and feeds as if they were communities.
But until Lemmy implements support for “user” posts and “user” following, we won’t see the majority of content of that type, coming from mastodon.
There’s already some funky interoperability that comes from the underlying structure of communities kind of being user accounts, where mastodon users can follow Lemmy communities, and post to communities by mentioning them. But it’s not pretty.