Follow these simple steps to start using Flatpak
Flatpak is installed by default on Manjaro 20 or higher.
To enable its support, navigate to the Software Manager (Add/Remove Programs)
Click on the triple line menu [or dots depending on the Desktop Environment] on the right, in the drop down menu select "Preferences"
Navigate to the "Flatpak" tab and slide the toggle to Enable Flatpak support (it is also possible to enable checking for updates, which is recommended).
To complete setup, restart your system. Now all you have to do is install some apps!