My work requires us to have full-disk encryption, so these are the steps I use.
The basic idea is to create a LUKS-encrypted partition which is used as an LVM Physical Volume.
The GRUB boot partition isn't encrypted, but everything else is.
These steps tested and working on 22.04 (jammy) and 20.04 (focal). For a target OS of Debian 11, I have used and tested this guide: https://linuxconfig.org/how-to-install-debian-on-an-existing-luks-container