Flatpak is a cross-distribution framework for distribution and running of sandboxed applications. XDG Desktop Portal provides Nuvola Apps with desktop integration. GNOME Software allows you to browse and install flatpaks in a more user friendly way. It is needed because elementary's AppCenter still doesn't support flatpaks.
All packages are available in the elementaryOS 5 Juno repository. No third-party package archive is required.
Alternatively, launch a Linux terminal emulator
sudo apt install flatpak xdg-desktop-portal-gtk gnome-software-plugin-flatpak
and press the Enter key to install these packages.
It is necessary to restart your computer to add Flatpak paths alongside the system ones and to activate the Ubuntu Software Flatpak Plugin. Otherwise, you may see strange errors. Unfortunately, just logging out and logging in is not enough.
The Flathub repository provides GNOME Platform runtime, which is used by Nuvola to share common dependencies with other Flatpak apps to reduce disk & memory usage. The Nuvola Apps repository contains Nuvola Apps Service and individual Nuvola web apps.
Refresh GNOME Software package cache: Launch GNOME Software → switch to the Updates screen → click the Refresh button in the top left corner.
Alternatively, launch a Linux terminal emulator and type following commands:
flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
flatpak remote-add --if-not-exists nuvola https://dl.tiliado.eu/flatpak/nuvola.flatpakrepo
Nuvola Apps Service is an optional background service that provides individual Nuvola apps with global shared resources such as a global configuration storage, global keyboard shortcuts, a HTTP remote control server, and a command-line controller. While this service is optional, the installation is recommended.
In order to install Nuvola Apps Service with GNOME Software, launch GNOME Software, search for Nuvola Apps Service and finally click the Install button.
Alternatively, launch a Linux terminal emulator,
flatpak install nuvola eu.tiliado.Nuvola and press the Enter key to install this package.