Debian

Install Flatpak
A flatpak package is available in Debian 10 (Buster) and newer. To install it, run the following as root:

sudo apt install flatpak
Install the Software Flatpak plugin
If you are running GNOME, it is also a good idea to install the Flatpak plugin for GNOME Software. To do this, run:

sudo apt install gnome-software-plugin-flatpak

If you are running KDE, you should instead install the Plasma Discover Flatpak backend:

sudo apt install plasma-discover-backend-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 the following in a terminal:

flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
Restart
To complete setup, restart your system. Now all you have to do is install apps!

Install Flatpak