Install flatpak
and flatpak-builder
using instructions from http://flatpak.org/getting.html
Make a folder, i.e. CubicSDR-flatpak and save com.cubicsdr.App.json there.
In that folder create a build folder and run flatpak-builder:
$ mkdir build
$ flatpak-builder --force-clean --repo=./repo ./build com.cubicsdr.App.json