You can learn how to install GNOME Flashback on Ubuntu 20.04 or 22.04 from a tutorial by Linux Capable.
How to Install GNOME Flashback on Ubuntu 22.04 or 20.04
Discover GNOME Flashback, a traditional and customizable desktop environment for Ubuntu 22.04 or 20.04 LTS. Install it easily and customize your panels, themes, and applets.
How to Install GNOME Flashback on Ubuntu 22.04 or 20.04 - LinuxCapable