add flathub to the (user) remotes flatpak remote-add --user --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
Build with flatpak-builder --user --force-clean --install --install-deps-from=flathub <BUILDDIR> sirEdit/de.marko10-000.sirEdit.json
Be careful, <BUILDDIR> and the current path should NOT be in the directory of the git.
Run the programm with flatpak run de.marko10_000.sirEdit