Disclaimer : this is just for testing, obviously if you have some mythical golden egg of a system don’t do this. If you have a test desktop or are just distro hoping its probably best to do it on that.
^ ^ ^ ^
Having said that, Someone might find this useful rather than hunting down the official iso
Prerequisites: Steam Installed in your normal desktop environment ( xfce,cinnamon,gnome )
Open a Terminal:
Type: sudo gedit /etc/apt/sources.list.d/steamos.list
Insert this text:
deb http://repo.steampowered.com/steamos alchemist main contrib non-free
deb-src http://repo.steampowered.com/steamos alchemist main contrib non-free
^
(( there was supposed to be a hash on the start of that line but it made the forum text go massive ))
Save the text.
Type in your terminal: sudo apt-get update
Type in your terminal: sudo apt-get install steamos-compositor steamos-modeswitch-inhibitor
Logout...
Select SteamOS session.
Note I have enabled desktop mode but for obvious reasons ( im not running gnome for one ) it wont actually logout of SteamOS so a reboot or shutdown is required after you have finished with SteamOS .. i didnt try ctrl + alt + f2 (f3,f4 etc.. ) that "might" of worked .
Hope this helps, its pretty much giving you SteamOS on your desktop which very much like Bigpicture but with some extra trimmings, the other thing it gives is their compositor so you can tell how your games would work on your existing hardware should you even want to turn your current rig into a full SteamOS box.
(although your drivers are probably more upto date already than the dedicated SteamOS ISO ones :P so performance here will probably be higher than a vanilla SteamOS install )