Tools, Customize, Keyboard button, change Categories to All Commands, search
for and select Highlight in the Commands window.
Click into "Press new shortcut key", do the keystrokes you want to apply.
Ensure that the "Save changes in" window has the template that you wish for
this shortcut to be available, or have Normal.dot if you want this option to
be available in all templates on your PC.
Hope this helps
DeanH