In other Office blog post, the team explains the steps to add a DRAFT watermark to a worksheet. Excel doesn't come with a built in way to add a watermark, but there's a pretty painless way to do it -- just follow the steps below:
In your worksheet, click the Insert Tab and in the Text group, click Header & Footer. Excel switches to Page Layout view, and your cursor appears in the center header box at the top of the worksheet page (under Header).
Click the Design tab, and in the Header & Footer elements group, click Picture. Find your copy of the DRAFT or CONFIDENTIAL .PNG file on your computer and add it to the header. You'll see that your worksheet header contains the word "Picture." To view the actual watermark, just click in any cell in your worksheet.
When you're viewing it, you'll be in Page Layout view. To return to the Normal view--the way you typically work in Excel--click the Normal button on the status bar at the bottom right of your screen.
(Note that headers are visible only in Page Layout view, the Print Previewwindow in Backstage view (File tab, Print), and the printed worksheet.)
Here is the end result:
If you want to adjust the position of the watermark or resize it, check out this article: Mimic a watermark in Excel.
Bonus: Here is a good list of Windows 8 keyboard shortcuts:
Windows key - Brings up the Metro start screen. You can start typing to search for an app, just like the Win7 start menu.
Win + B - Switch to the (classic) Windows desktop and select the tray notification area.
Win + C - Brings up the Charms menu, where you can search, share, and change settings.
Win + D - Brings up the old Windows desktop.
Win + E - Launch Windows Explorer with Computer view displayed.
Win + F - Brings up the Metro File search screen.
Win + H - Opens the Metro Share panel.
Win + I - Opens the Settings panel, where you can change settings for the current app, change volume, wireless networks, shut down, or adjust the brightness.
Win + J - Switches focus between snapped Metro applications.
Win + K - Opens the Devices panel (for connecting to a projector or some other device)
Win + L - Lock PC and return to Lock screen.
Win + M - Minimize all Windows on the desktop
Win + O - Locks device orientation.
Win + P - Choose between available displays.
Win + Q - Brings up the Metro App Search screen.
Win + R - Switch to the (classic) Windows desktop and display the Run box.
Win + U - Switch to the (classic) Windows desktop and launch the Ease of Access Center.
Win + V - Cycles through toasts.
Win + W - Brings up the Metro Settings search screen.
Win + X - Launch Start Menu.
Win + Y - Temporarily peek at the desktop.
Win + Z - Opens the App Bar for the current Metro application.
Win + Page Up / Down - Moves tiles to the left / right.
Win + Tab - Opens the Metro application switcher menu, switches between applications.
Win + , (comma) - Aero Peek at the desktop.
Win + . (period) - Snaps the current Metro application to one side of the screen. (Right side)
Win + Shift + . (period) - Snaps the current Metro application to the other side of the screen. (Left side)
Win + Space - Switch input language and keyboard layout.
Win + Shift + V - Cycles through toasts in reverse order.
Win + Enter - Launches Narrator
Win + Arrow Keys - Switch to the (classic) Windows desktop and enable Aero Snap