1. Follow the Flatpak installation instructions for your distribution.
2. Add the Flathub repository - with many distributions, you can download the the repository file and install it with GNOME Software. Alternatively, from the command line, run:
$ flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
3. That's it! You can now install apps from an app center, by browsing them online, or through the flatpak command line.
Security or legal issues can be reported to the Flathub maintainers.