Here was the best tutorial I could find for it.

You can create posts in Lemmy communities by tagging the community account
(It should be a public post)
If the community account is tagged with @, it will share the post and the post will also appear in the forum.
For example, this is a post I created from Mastodon in feddit’s #Tischtennis forum: metalhead.club/@caos/112749905… … and this is how it is displayed in Lemmy: feddit.org/post/556495

The only thing to note from Mastodon and Akkoma etc. is: The beginning of the post/the first paragraph becomes the title of the forum post, as Mastodon does not have a heading field. (see also: Instructions Creating a post from Mastodon)

So it is best to start the post like this (see image 1):

This is my headline (as descriptive a title as possible)
@community@lemmy-instance
This is the further text, link etc.
if necessary a picture (only in the initial post a picture is transferred from Mastodon to Lemmy, between Lemmy and Friendica all images in answers are transferred in the meantime)