Most people won't go that route. Instead, they'll turn to third parties that have posted copies of their own builds of the source code online. The folks at GDGT.com built their own virtual disk image of Chromium OS, and GeekLad created his own VirtualBox appliance . It's also possible to convert these images into a USB-bootable image (as GDGT.com has done), so you can try out Chromium OS on any computer that boots from a USB.
Booting And Logging In
The key word is "try." Don't be surprised if you boot Chromium OS on some hardware and are greeted with a black screen. Part of this is because it's still an alpha-stage project; part of it is because you might be using hardware that Chromium OS isn't designed to support (or supports only shakily right now). Virtual machines -- like the aforementioned VirtualBox -- seem to produce the most consistent results, since the environment within the VM is rigorously controlled.
When you first boot Chromium OS, you should see a login screen very quickly. Boot times on Chromium are fast; if you don't see something within ten to fifteen seconds, odds are your hardware is not compatible with the current build. Google maintains a list of hardware with known behaviors , so if you have access to anything on that list you can improve your odds of booting and running by using something documented there.
You can log in one of two ways: with a Google or Gmail account (if you have one), or by using a local login named chromos (blank password). In order for any login other than chromos to work, you need to have network connectivity -- so if you're dubious about the state of your networking on your test hardware, you can always log in with chromos first and check connectivity once you're inside. Note that any login name has "@gmail.com" automatically appended to it; this is normal.
The OS Itself
After logging into Chromium OS, more than one person has said "That's it?" Yes, that's it -- you'll see a browser, one that looks a great deal like Google Chrome as we've come to know it on the PC. And yet, there are a few key things that make all the difference.
The "Simon"-like icon on the top left side of the browser works something like the Start or Windows-logo button on a conventional PC. Click it and you'll be taken to a landing page where many common Web services -- some Google, some third-party -- are available.
This page isn't directly customizable yet, but you can get an idea of how it works by launching some things. Contacts, for instance, pops open a Google Talk window; this little pop-up window format is used for many other on-the-spot functions.
Since the vast majority of what you do in Chromium OS is going to be in a browser, window management is at a minimum. Apart from adding or deleting tabs in the browser itself, there's an Aero Peek / Exposé -like multi-window view which you can fire up by hitting F12. From there you can open entirely new browser windows or switch between them. Also, pressing Ctrl-Alt-Del brings up the Task Manager -- no, not the Windows version, but the one you find in Chrome via Shift-Esc and "Stats for nerds" (i.e., about:memory).