This is my personal flathub cheatsheet
Build it
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
./build.sh
Test it
flatpak-builder --run build_x86_64 org.nomacs.ImageLounge.json nomacs
push a commit with Update org.nomacs.ImageLounge.json
to update the package on flathub