Launch a Linux terminal emulator,
type flatpak install nuvola eu.tiliado.NuvolaAppYoutube and press the Enter key to install this package.
Then log out and log in back to let your desktop environment (Unity, GNOME, etc.) find out that
a new application has been installed. Otherwise, you may see "Unknown Application Name" in the menu bar,
mismatched application icons and other funny bugs.
You should be able to find YouTube in a list of installed applications. If not, you may have forgotten
to log out and log in and your desktop environment (Unity, GNOME, etc.) has not find out yet
that a new application has been installed.
Alternatively, you can launch YouTube
from a terminal:
flatpak run eu.tiliado.NuvolaAppYoutube