Thread title goes into the top line. It's mandatory. Top line/paragraph must be only the thread title. No post text.
Line below: Mention the Lemmy community that you want to post to. The Lemmy community must be mentioned between the title and the post text, as in, under the title and above the post text. Nothing else must be in this line/paragraph.
Below that: post text in as many paragraphs as you need.
No further mentions anywhere else in the post.
Also, no hashtags. They won't break anything, Lemmy simply doesn't support hashtags. But some 99% of all Lemmy users come from Reddit (as opposed to almost everyone on Mastodon coming from Twitter), Reddit doesn't have hashtags, and so they aren't used to hashtags. Lemmy's culture is basically Reddit's culture, and Mastodon's culture is irrelevant and invalid on Lemmy.