If you're using Microsoft Word, it has a built-in watermark feature.
In Word versions prior to Word 2007:
1. Click on the Format->Background->Printed Watermark menu item. If Printed Watermark is disabled (grayed out), the document is probably in Web page view. You can't use watermarks on Web pages.
2. In the dialog box, select Text Watermark.
3. Put the text you want in the Text combination box.
4. Select Diagonal or Horizontal.
5. Select Semitransparent
6. Click OK.
7. Click the Tools->Options menu item.
8. Click the Print Tab.
9. Under Include with Document, check the checkbox for Drawing Objects.
10. Click OK.
In Word 2007:
1. On the Page Layout tab on the ribbon, in the Page Background section, click Watermark.
2. Click Custom Watermark.
3. In the dialog box, select Text Watermark.
4. Put the text you want in the Text combination box.
5. Select Diagonal or Horizontal.
6. Select Semitransparent
7. Click OK.
8. Click the Office Button.
9. Click the Word Options button.
10. Click the Display tab.
11. Under Printing Options, check Background Colors and Images
12. Click OK.
That should do it.