I have written a detailed tutorial in three parts, which may help you a little.
Since some files will be changed, don't forget to make a backup beforehand. These changes will be lost with the next update, but I think that with FlatPress 1.3 the errors have already been fixed and if not - then there will be a new guide from me

The tutorial is in German, but you can use GoogleTranslator to display the pages in your language. With a bit of luck, it will also be reasonably understandable.
So, enough of the words - actions only bring us further:
Part 1: https://madekozu.de/beitrag.php?id=21
Part 2: https://madekozu.de/beitrag.php?id=22
Part 3: https://madekozu.de/beitrag.php?id=23