Flatpak applications can be installed on Chrome OS with the Crostini Linux compatibility layer. This is not available for all Chrome OS devices, so you should ensure your device is compatible before proceeding. A list of compatible devices is maintained here.

1. Enable Linux support

   Navigate to chrome://settings, and scroll down to Linux (Beta) and turn it on. Chrome OS will take some time downloading and installing Linux.

2. Start a Linux terminal

   Press the Search/Launcher key, type "Terminal", and launch the Terminal app.

3. Install Flatpak

   To install Flatpak, run the following in the terminal:

   
         $ sudo apt install flatpak
       

   A more up to date flatpak package is available in the Debian backports repository.

4. Add the Flathub repository

   Flathub is the best place to get Flatpak apps. To enable it, run:

   
         $ flatpak --user remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
       

5. Restart

   To complete setup, restart your system. Now all you have to do is install some apps!