Here is how you can import the NixOS module and Home Manager module respectively.
When imported, new options will be made available under services.flatpak. Please refer to the Module definition for configuration options.
Important
The stable branch is versioned. You will get notified when it gets obsoleted read here for more info.
Note
Your setup must be able to hold the size of your flatpak installation at least twice.
Note
Contrary to popular belief, this module was never intended to implement "generational rollbacks". The unique directory layout was created to enable atomic updates for flatpak.