How to Install Flatpaks
Why you should think twice before doing so: Alternatives
A flatpak is a supposedly self-contained application: that is a package which contains the application, itself, together with all its dependencies. Great idea, except:
(a) to actually install the flatpak application your operating system has to provide the flatpak 'framework' which means first installing that 'framework'.
(b) being self-contained, there's a good chance that many of the dependencies will be duplicates of files found elsewhere on your system-as-a-whole; [Not much of a problem if you aren't concerned about diskspace]; and
(c) flatpak applications are not built for Puppys. 'Self-Contained' relates to the operating systems they were built and tested to run under. To run under Puppys you may also have to install dependencies which came as built-ins in the operating systems flatpaks were designed for.
(d) and this one is important, OOTB Puppys do not support even installing the flatpak 'framework'.
So the first thing I would ask is "Is an application provided as a flatpak the only way to obtain that application?" Has someone already built a version of the application you want for your Puppy? Look in Menu>Setup>Quickpet, if your puppy has it. Then look on the threads relating to your Puppy; then in the appropriate Additional Software Sections.
Next, did the publisher of the flatpak also publish an AppImage? Many do. These are also 'Self-Contained' but do not require the flatpak framework. If you're lucky you can just download it, make it executable and run it. If you're not that lucky you'll still have the problem mentioned in (c) above. But it's likely that you'd also have that problem with the corresponding flatpak package.
Next, check if the application is available and can be installed via Puppy Package Manager. It makes little sense to install an unnecessary 'framework'.
How you can
Next, install pkg-cli, https://forum.puppylinux.com/viewtopic.php?t=4802 or pkg2, mistfire’s Pkg2: The fork of scottman's pkg, viewtopic.php?p=51728#p51728. Like apt these small applications make it much easier than PPM to add repositories and install applications from them together with dependencies. You will need one or the other in order to install the flatpak ‘framework’. But, before doing that, see if it’s now possible to install the application directly without also installing the flatpak framework.
If all else has failed, follow rockedge’s instructions here, viewtopic.php?p=3651#p3651, to use pkg-cli or pkg2 to install the flatpak ‘framework’.
Edit: 29 March 2024. As of this date when following rockedge's instructions be aware that newer versions for installing flatpak for 'Ubuntu-binary-compatibles' can be found at https://launchpad.net/flatpak/+archive/ubuntu/stable