Click the green new item button and select “Letter”. Use Author for the sender of the letter. To add a recipient click the + sign on the author line in the right column. This will create an additional Author line. If you click the triangle to the left of the new author field you will be able to change Author to Recipient. Enter the type of letter (memo, telegram, etc.) in the “Type” field. On entering archival information, see How do I add an archival or other unpublished source?
If you have a scan of the letter (e.g., a PDF or an image), you could add it as an attachment to the Letter item.