Click on the the template. At the bottom right of the dialogue you'll see two options: 'Document' and 'Template'. If you want to use the letterhead to send as a document or letter, choose 'Document.' If you want to edit the template, choose 'Template.' When you choose 'Document,' it gets saved as an MS Word document.
Click on 'Insert,' again at the top of MS Word. When the menu appears, let your mouse arrow hover over 'Picture.' Here you'll have several options. You can create your letterhead on the fly using some of those options. However, for our purposes we'll opt to choose 'From File,' since we already have a ready graphic. Click on the option, 'From File,' and a dialogue box opens. Locate your graphic and then click on it and then click the 'Insert' button. Your graphic will then appear inside the dotted box.