it depends -if you just want Word to open up and thats all -
use the button wizard -
add a new button and one of the options is application select word and done
if its a set word doc lets say sample.doc then use hyper link use the help option (I've copied this below) - this works great used it plenty of times
hope this helps
Assign a hyperlink to a toolbar button or menu command
When you assign a hyperlink to a toolbar button or menu command, the hyperlink replaces the command currently assigned to that button or menu command. You cannot assign a hyperlink to a button that displays a list or menu when the button is clicked.
Display the toolbar you want to change.
If the button you want to assign a hyperlink to is not on a toolbar, add a button to the toolbar.
If the menu command you want to assign a hyperlink to is not on a menu, add the command to a menu.
On the Tools menu, click Customize.
Right-click the toolbar button, point to Assign Hyperlink on the shortcut menu, and then click Open.
Under Link to, click the source you want to link to.
Select the options you want.
For Help on an option, click the question mark and then click the option.