There are no installation images for sid, but we can choose either stable or a daily built (testing). If you are absolutely certain that you will have connection to the internet while installing, i.e. in VirtualBox, with a cable or with the correct firmware available for wifi, you should use the netinstall image, otherwise a cd image that will give you the base system that will allow you to work and get online.

We may need jigdo to download our image, so read the Debian mini how-to if you are not familiar with it.

When installation is complete (including the choice to have a GUI or not), I first add emacs, mc and sudo with apt and add my user to the sudo group.

Now I open /etc/apt/sources.list (usually with nano), and change any instances of stable, testing or distribution name into sid, add contrib and non-free and comment out security lines (which don’t work for unstable.

This leaves us with

deb http://deb.debian.org/debian/ unstable main contrib non-free
deb-src http://deb.debian.org/debian/ unstable main contrib non-free

Now we run sudo apt update and sudo apt dist-upgrade and we have a running Debian sid.