Pop!_OS 20.04 has Flatpak installed and Flathub configured by default. The Pop!_Shop can be used to install flatpaks.
For older versions of Pop!_OS, see the instructions below.
To install Flatpak on Pop!_OS 19.10 and earlier, simply run:
sudo apt install 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!
Note: graphical installation of Flatpak apps may not be possible with Pop!_OS 19.10 and earlier.