Essentially, the title of the post/thread comes first.

Then comes a blank line.

Then come the mentions, all in one line. They must always start with the Lemmy community, and you can only mention one Lemmy community. If you want to crosspost to a Friendica group, a Hubzilla forum or a (streams) group, it comes next, and AFAIK, that can only be one, too. Guppe groups come afterwards, as many as you want.

Then comes another blank line.

And then comes your actual post.

Another blank line.

Lastly, hashtags so that Mastodon has them where it expects them.