You do not say which version of Word you're using, or whether you need
onscreen functionality.
For a simple checkbox that you plan to print, (assuming Word 2003 or
earlier), choose Insert - Symbol - Symbols tab. There are a variety of
characters that can be used for checkboxes -- set the Font to Wingdings or
Webdings--there are several in each of those fonts.
If you want onscreen functionality, you would do that using Word's Forms
feature. If you're using Word 2003, right-click any toolbar and choose
Forms. The checkbox is the 2nd item.
To actually have checkbox functionality, you would need to protect the
document as a Form. I suggest that you press F1, type forms, and search for
help to get details about how to do this.
