We follow CMS guidance on the usage of em dashes. See CMS 6.82–6.89 for rules regarding the use of em dashes.
On the Web
Treat an em dash as a word, with a space on either side. Select the em dash from the symbols palette, or in Windows, use the Alt code on your numbers keypad on your keyboard (ensure that the “Num Lock” key is released): ALT + 0151.