Detailed installation instructions for TWRP + ROM + Magisk in 1st post here.
For Redmi Note 9 Pro (Joyeuse):
For Redmi Note 9S (Curtana): https://xiaomi.eu/community/threads/custom-twrps-for-redmi-note-9s-curtana-2-versions-already.56943/
But everybody says in Telegram it's necessary to format data. That's my doubt, don't know if wipe cache would be enough or not.