These notes are based on Qubes 3.0. Trying out 3.1 RC1 shall also be done.

Install Debian Templates

    qvm-clone debian-8 debian-9
    qvm-run -a debian-9 gnome-terminal
    // adjust /etc/apt/sources.list and /etc/apt/sources.list.d/*.list
    // unclear atm whether to use qubes-r3.list or qubes-r31.list
    apt-get update && apt-get dist-upgrade

    deb [arch=amd64] http://deb.qubes-os.org/r3.1/vm scretch-testing main

Using 4.x kernels with Qubes 3.0

Qubes 3.0 comes with a 3.18 kernel by default. An update to 4.x kernels is easiest by installing the kernel package from the upcoming Qubes 3.1 release:

sudo qubes-dom0-update --releasever=3.1 kernel

or

sudo qubes-dom0-update --enablerepo=qubes-dom0-unstable kernel