Follow these simple steps to start using Flatpak
To install Flatpak on Kubuntu, simply open Discover, go to Settings, install the Flatpak backend and restart Discover.
To integrate Flatpak support into the Plasma System Settings, simply run:
Flathub is the best place to get Flatpak apps. To enable it, open Discover, go to Settings and add the Flathub repository.
To complete setup, restart your system. Now all you have to do is install some apps!