Slackware

Install Flatpak
A Flatpak package is available on SlackBuilds. To install it (assuming you use Sbopkg/sqg), first create the queuefile and run the following as root:

sqg -p flatpak

Then compile it. Load the queuefile, press 'Q', and wait:

sbopkg -i flatpak
Add the Flathub repository
Flathub is the best place to get Flatpak apps. To enable it, run:

flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
Restart
To complete setup, restart your system. Now all you have to do is install apps!

Note: graphical installation of Flatpak apps may not be possible with Slackware.

Install Flatpak