Open the word document that you want to add a watermark to. Go to the Page Background section and click Watermark.
Now you can either choose one of the default watermarks, or add a custom image.
Click on Custom Watermark as the bottom of the menu.
Now you can add your own custom text watermark or browse for an image (if you already have one ready). You will be able to add a custom font, colour, layout, and change the size of the watermark.