First of all, Installing SteamOS will erase all data on all connected hard drives as far as I know. There is no dual-boot option, not even any option to select partition scheme. So do not install it on a computer where you have data you don't want to lose. It will also require UEFI firmware, which most newer motherboards should have.
SteamOS also created a recovery partition the first time you boot it up after installation. I do not know if it is safe to install a second OS later for dual booting, since it could be that SteamOS would take control of all partitions and hard drives if the recovery partition is updated.
Official instructions and download is available from here:
http://store.steampowered.com/steamos/buildyourown
The zip is a regular zip file, you should be able to open and extract it with just about any archive program. Once the USB is ready, you can simply extract the archive to the USB drive.
When you partition a storage device, you need to select what partition table to use. The two most commonly used are mbr and gpt. I think you can do this in gparted, you need to select to
create new partition table. Then you need to select mbr. I have seen some programs label mbr as "ms-dos partition table", so look for that if mbr does not seem to be available. Personally I use fdisk/cfdisk and gdisk/cgdisk for partitioning, but those are only terminal programs without a graphical interface.
You must also remember to make the new partition bootable. And then format it to FAT32.
I was also surprised that they don't supply an iso file for this, but perhaps it makes it easier to make frequent modifications to the install medium when it is supplied in a zip file.
Just ask if you get stuck. But do keep in mind, installing SteamOS will erase all data on your hard drives.
I have had a little look around in the operating system. It is indeed made to only run in Steam Big Picture mode. Enabling the desktop from within Steam settings will also give you a warning that using the desktop is unsupported. This is probably just to keep Valve from having to provide support for computer newbies possibly breaking the Linux OS. All packages required for PlayOnLinux are also not present. I am considering enabling Debian8 repositories to get all the packages, or if I should just download them manually. If Aptitude or apt-get can list all packages that were not installed from repositories, then I can more easily track my own modifications to the operating system. So I will have to read up on those package managers before I decide.
Personally I do not have the biggest interest in SteamOS. If you install Linux Steam on any Linux distribution, then enable Big Picture mode, you get the same experience. But I am interested in the possibility of integrating PoL shortcuts into the SteamOS interface, and I want to test that within SteamOS to ensure that things would work as intended.
