Ubuntu

Install Flatpak
To install Flatpak on Ubuntu 18.10 (Cosmic Cuttlefish) or later, open the Terminal app and run:

sudo apt install flatpak
Add the Flathub repository
Flathub is the best place to get Flatpak apps. To enable it, download and install the Flathub repository file or run:

flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
Install Bazaar
With Flathub enabled, install Bazaar directly from Flathub by running:

flatpak install flathub io.github.kolunmi.Bazaar

Then launch it from your desktop to install Flatpak apps graphically.
Restart
To complete setup, restart your system. Now all you have to do is install apps!

Install Flatpak