Follow these simple steps to start using Flatpak
To install Flatpak on Ubuntu 18.10 (Cosmic Cuttlefish) or later, simply run:
$ sudo apt install flatpak
With older Ubuntu versions, the official Flatpak PPA is the recommended way to install Flatpak. To install it, run the following in a terminal:
$ sudo add-apt-repository ppa:flatpak/stable
$ sudo apt update
$ sudo apt install flatpak
The Flatpak plugin for the Software app makes it possible to install apps without needing the command line. To install, run:
$ sudo apt install gnome-software-plugin-flatpak
Note: the Software app is distributed as a Snap since Ubuntu 20.04 and does not support graphical installation of Flatpak apps. Installing the Flatpak plugin will also install a deb version of Software and result in two Software apps being installed at the same time.
$ flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
$ flatpak remote-add --if-not-exists ramatak https://download.ramatak.com/ramatak.flatpakrepo
To complete setup, return to the download page, and follow the instructions.!