I made a brief guide here: https://hachyderm.io/@maegul/110483509521476095
What’s more, is it is posted to the test community on lemmy.ml, so you can find there too (link: https://lemmy.ml/post/1142168). A few people replied so you can see the whole federation thing happen too.
It is really the only way to fuse the two systems (posting from masto to lemmy), so please do this as much as you can!