Install KDE runtime
If you don't have KDE runtime installed, you should install it according to instructions.
To install QupZilla Nightly builds, use following commands:
wget https://qupzilla.com/flatpak/qupzillaflatpak.asc flatpak --user remote-add qupzilla https://qupzilla.com/flatpak/repo/ --gpg-import=qupzillaflatpak.asc flatpak --user install qupzilla org.qupzilla.QupZilla
This will install QupZilla for the current user. You should then see it in applications menu as QupZilla (Nightly).
It is still experimental version, but it should work fine. Please try it and report any issues you found.