To easily insert a letterhead or signature, you can create a Snippet.
1. In More > Resources > Letter Templates select New Template.
Fill in the details, tick the Add to Snippets checkbox
2. Use any formatting tools you'd like to create your Letterhead.
3. When you are happy with the layout of your letterhead, click File and Save.
4. Ensure that Add to Snippets is ticked and Save the document.
5. Now, whenever you create a Letter, simply navigate to the Snippets drop down to insert your letterhead. Snippets can be inserted at any point, so you can also create a Signature Snippet to add to the end of your letter.