This is not simple.
See How to set the “tab order” of a Word form by Dave Rado and Mark Tangard.

With Word 2007 and later, though, you may want to consider migrating to Content Controls which are far more flexible and powerful. If you do that, this page would assist. Content Control Grouping & Navigation by Greg Maxey (There are free utilities available to convert legacy form fields to Content Controls if you are interested.)