If you regularly use certain formatting or other commands in Word, it can be very convenient to bind these to hotkeys, rather than to use the buttons on applications' toolbars every time. To do this:

1. Select Tools > Customize
2. Click the Keyboard button
3. Select the category within which your function lives
4. Select the function itself
5. Click in the "Press New Shortcut Key" box and press a new shortcut key combination. If this combination is already in use, you'll be told what it's for and can decide to change it or not.
6. To use your selected hotkey, press "Apply"