make sure you’ve added the debian testing repositories to your /etc/apt/sources.list
deb http://ftp.debian.org/debian/ testing main contrib non-free
deb-src http://ftp.debian.org/debian/ testing main contrib non-free

update the package list
sudo apt-get update

then install equivs (this is used to create a fake jockey-common package) and realpath
sudo apt-get install equivs realpath

download the steam debian installer from here.

install the installer
sudo dpkg -i steam-debian_1.0.0.39-9-2_all.deb

then run steam-debian and then steam

it’ll update itself and you should be up and running…