It is important to follow the steps in Shauna's article all the way through. Automatic numbering in Word is broken, especially when applied using the buttons. In one-use documents this usually is not a problem. It is when documents are edited that the problems usually pop up.

You need to attach all levels you might ever use to appropriate styles at one time. If you want to understand what is going on rather than just follow the steps, look up John McGhie's article listed in the chapter below. AFAIK, Shauna's steps integrate John's advice very well.

Numbering in Microsoft Word