Press Ctrl+D or choose Font from the Format menu. (If you are using Word 2007, press Ctrl+D or click the Home tab of the ribbon, then click the small control at the bottom-right of the Font group.) Word displays the Font tab of the Font dialog box. (Click here to see a related figure.)
Hold down Alt+Ctrl and, at the same time, press the plus sign on the numeric keypad. The mouse pointer turns into a clover symbol.
Click on the Strikethrough check box in the Font dialog box. (As you move the mouse pointer to get ready to click, the mouse pointer may change back to an arrow instead of a clover; this is OK.) When you click, Word displays the Customize Keyboard dialog box with the insertion point blinking in the Press new Shortcut Key box. (Click here to see a related figure.)
Type whatever shortcut key you want to use for the strikethrough format. Just hold down whatever combination of the Alt, Ctrl, and Shift keys you want, and then press the desired key to go with that combination. If the combination is already taken, that information shows just below the Customize Keyboard dialog box, and you can then change to a different shortcut key. (A good combination to consider is Alt+Shift+S or Ctrl+Alt+S, neither of which are used in a default installation of Word.)
Click the Assign button. The shortcut key is now assigned to apply strikethrough formatting.
Click Close to dismiss the Customize Keyboard dialog box.