Follow these simple steps to start using Flatpak
To install Flatpak, run the following:
$ sudo nix-env -iA nixos.flatpak
Flathub is the best place to get Flatpak apps. To enable it, run:
$ flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
To complete setup, restart your system. Now all you have to do is install some apps!