Follow these simple steps to start using Flatpak
Flatpak applications can be installed on ChromeOS with the Crostini Linux compatibility layer. This is not available for all ChromeOS devices, so you should ensure your device is compatible before proceeding. A list of compatible devices is maintained here.
Navigate to chrome://os-settings, and scroll down to Developers and turn on Linux development environment. ChromeOS will take some time downloading and installing Linux.
Press the Search/Launcher key, type "Terminal", and launch the Terminal app.
To install Flatpak, run the following in the terminal:
A more up to date flatpak package is available in the Debian backports repository.
Flathub is the best place to get Flatpak apps. To enable it, run:
To complete setup, restart Linux. You can do this by right-clicking terminal, and then clicking "Shut down Linux". Now all you have to do is install some apps!