When you're done, you can click on the 'Close' tab on the floating bar. The items in the header and footer will appear greyed out, but don't worry; this only means the they have been set. The cursor will then appear on the first line where you can begin typing your letter.
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.