1. It starts, ironically, in Desktop mode, which you can reach by clicking/tapping the Desktop tile or pressing Win-D (that's the Windows key and the letter D).
2. Right-click in any open spot on the desktop, then choose New, Shortcut.
3. We're going to make a Shutdown shortcut, so in the location field, type shutdown /p and then click Next.
4. Choose a different name for the shortcut if you want, or leave it as "shutdown," then click Finish.
5. Right-click the new shortcut and choose Pin to Start.
Presto! You're done. The next time you return to the Start screen (aka Metro), you should see a Shutdown tile. One click or tap and your machine should immediately shut down.
You can also use this method to create a reboot tile. Just substitute the following text in step 3: shutdown /r /t 0. (That's a zero at the end, and you shouldn't include the period after it.)
Note that using the reboot tile will immediately reboot Windows, with no second-chance warning.